Ch 9 - Conversion and Calculations

The ratio formula can be used in calculating dosages. For example, the prescriber orders heparin 10,000 units SC (subcutaneously); the dose on hand is 40,000 units/mL.

(H) 40,000 units / (V) 1 mL = (D) 10,000 units / X cross multiply get X = 10,000/40,000 reduce to 1/4 (4 goes into 100) 0.25 so patient gets 0.25 mL of heparin SC

Conversion Factors

1 kg = 2.2 lb 1 gallon = 4 quart 1 tsp = 5 mL 1 inch = 2.54 cm 1 L = 1,000 mL 1 kg = 1,000 g 1 oz = 30 mL = 2 tbsp 1 g = 1,000 mg 1 mg = 1,000 mcg 1 cm = 10 mm 1 tbsp = 15 mL 1 cup = 8 fl oz 1 pint = 2 cups 1 L = 1.057 qt 1 lb = 16 oz 1 tbsp = 3 tsp 1 cc = 1 mL 2 pints = 1 qt 8 oz = 240 mL = 1 glass 1 tsp = 60 gtt (drops) 1 pt = 500 mL = 16 oz 1 oz = 30 mL 4 oz = 120 mL

To prevent medication errors, the technician should remember the following:

1. Right patient ~ Check the name on the order and the patient. ~ Use 2 identifiers. ~ Ask patient to identify himself/herself. ~ When available, use technology (example, bar-code system). 2. Right medication (drug) ~ Check the medication on the label of the bottle. ~ Check the name of the medication on the order (Rx). 3. Right dose ~ Check the order (Rx) and bottle. ~ Confirm appropriateness of the dose using a current drug reference. ~ If necessary, calculate the dose and have another tech calculate the dose as well. 4. Right route ~ Check the order and appropriateness of the route ordered. ~ Confirm that the patient can take or receive the medication by the ordered route. ~ Drugs are introduced into the body by several routes. • Taken by mouth (orally) • Given by injection into a vein (intravenously, IV) • into a muscle (intramuscularly, IM) • into the space around the spinal cord (intrathecally) • beneath the skin (subcutaneously, sc) • Placed under the tongue (sublingually) • between the gums and cheek (buccally) • Inserted in the rectum (rectally) or vagina (vaginally) • Placed in the eye (by the ocular route) • the ear (by the otic route) • Sprayed into the nose and absorbed through the nasal membranes (nasally) • Breathed into the lungs, usually through the mouth (by inhalation) or mouth and nose (by nebulization) • Applied to the skin (cutaneously) for a local (topical) or body wide (systemic) effect • Delivered through the skin by a patch (transdermally) for a systemic effect 5. Right technique ~ Before or after meals or with food. ~ In the A.M. or P.M. ~ Avoid dairy or acidic foods/drink 6. Right documentation ~ Chart the time, route, and any other specific information as necessary. 7. Right time/frequency ~ Check the frequency of the ordered medication.

1. Heparin 2. Antibiotics 3. U-100 insulin 4. Potassium Chloride

1. USP 20,000 to 40,000 units every 24 hours 2. mg/mL 3. USP 100 units/mL 4. milliequivalents/mL

Fraction and decimal fractions

4/10 is 0.4 Because 10 has one zero, the decimal point of 4 is moved to the left once. 17/100 is 0.17 Because 100 has two zeros, the decimal point of 17 is moved to the left twice. 334/1000 is 0.334 Because 1000 has three zeros, the decimal point of 334 is moved to the left three places.

(proportion) The solution strength on hand is 50 mg per milliliter. A dosage of 180 mg is ordered.

50 mg / 1 mL = 180 mg / X mL X = 180/50 = 3.6 mL This answer is logical because 3.6 mL is larger, and the dosage ordered (180 mg) is larger than the solution on hand (50 mg/mL), and must be contained in more than 1 mL.

The strength available is 1200 mcg per milliliter. A dosage of 800 mcg has been ordered.

800mcg/1200mcg x 1mL = X mL So, 800/1200 = 1.5 x 1 mL =0.66 round up to 0.7mL

Standard numerical numbers.

Arabic numbers

The physician's order is for ampicillin 0.5 g PO tid. (by mouth three times a day) The drug available is ampicillin 250 mg capsules.

Both the dosage of the drug ordered and the dosage available are in the metric system; however, the units of measurement are different. They must be converted. To convert grams to milligrams 0.5 g = 0.500 mg = (move the decimal point three spaces to the right) = 500 mg (ordered) (250 mg on hand) D/H x V = 500/250 = 2/1 x 1 capsule = 2 capsules 250 goes into 500 twice so patient needs 2 capsules per dose.

You have on hand a dosage strength of 40 mg in 1 mL. A dosage of 30 mg is ordered. How many milliliters are necessary to administer this dosage?

D (desired dosage) = 30 mg H (on hand) = 40 mg in V (vehicle) 1 mL 30/40 = 0.75 mL See pg 160

A dosage of 0.4 mg is ordered. On hand is 0.35 mg in 1.5 mL.

D = 0.4 mg / H = 0.35 mg in (V) 1.5 mL So, 0.35 goes into 0.4 (move the decimal two places to make it 35/40 = 1.143) 1.143 x 1.5mL = 1.7 mL

List the most common types of intravenous solutions.

Dextrose ~ D Lactated Ringer's ~ LR Normal saline ~ NS Saline ~ S Ringer's lactate ~ RL Water ~ W

Describe the relationship between decimals and fractions.

Fractions and decimals are similar because they both are ways to express partial numbers. Fractions can be expressed as decimals by performing the division of the ratio. (For example, 3/4 is equivalent to 3 divided by 4, or 0.75.) To obtain a decimal from a common fraction, divide the numerator by the denominator, and place a decimal point in the proper position on the answer line. 3/5 = 3 divided by 5 = 0.6 Decimals can also be expressed as fractions in terms of tenths, hundredths, thousandths and so on. (For example, 0.327 is equivalent to 327 thousandths, which is equivalent to 327/1,000.)

(proportion) A solution strength of 10 mg per milliliter is needed to prepare a dosage of 15 mg. The solution strength available (10 mg/mL) provides the known ratio. The dosage to be given is the incomplete ratio (15 mg). X represents the milliliters that will contain 15 mg.

Notice that both numerators are expressed as milligrams while both denominators are expressed as milliliters. The ratios in a proportion must be written using the same sequence of measurement units. 10 mg/ 1 mL = 15 mg/ X mL The first fraction is a complete ratio that expresses the drug strength. The second fraction is an incomplete ratio that expresses the dosage to give. By cross-multiplying, it is easy to see that: X = 1 x 15/10 (aka 15/10) = 1.5 mL The ordered dosage of 15 mg is contained in 1.5 mL. It is important to double-check your calculations and to assess whether each answer is logical. In this example, since 1 mL contains 10 mg, you will need a larger volume than 1 mL to obtain 15 mg. The answer, 1.5 mL, is larger, so it is logical. Although this does not verify that the calculation is correct, it indicates that you set up the proportion correctly and cross-multiplied sufficiently.

Decimals

Numbers to the left of the decimal point are whole numbers; numbers to the right of the decimal point are fractions with denominators of only 10, 100, 1,000, 10,000, etc. Decimal fractions or decimals are used with the metric system, which is most often used in the calculation of drug dosages. Each decimal fraction consists of a numerator that is expressed in numerals, a decimal point placed so that it designates the value of the denominator, and the denominator, which is understood to be 10 or some power of 10. In writing a decimal fraction, always place a zero to the left of the decimal point, so that the decimal point can readily be seen.

Explain standardized units of drug dosages.

Several drugs that are obtained from animal sources can be standardized in units according to their strengths rather than on weight measures such as milligrams and grams. Some hormones such as insulin are too complex to be completely purified to obtain the exact weight of the drug per unit of volume. Therefore, insulin and many other drugs are measured in units for parenteral administration. The labels of such medications indicate how many units are needed per milliliter.

(proportion) You have a 200 mg per 1.5 mL solution and need to prepare a dosage of 100 mg:

Shortcuts may be used to simplify the math in these problems. To do this, after cross-multiplying, immediately divide the number in front of X. You can also reduce numbers to their lowest common terms whenever possible. 200 mg / 1.5 mL = 100 mg / X mL 200 X = 1.5 x 100 (= 150) 150/200 = 0.75mL A dosage of 100 mg requires fewer milliliters than the 200 mg per 1.5 mL strength available. Therefore, the smaller answer, 0.75 mL, is logical.

The available drug strength is 500 mg in 2 mL. How many mL are needed to prepare a 300 mg dosage?

Solving for mL: mL = 2 mL/500 mg x 300 mg / 1 2 mL x 300 mg = 600 mg per mL / 500 mg (cancel the mg in both numerator and denominator) therefore 600 mL / 500 = 1.2 mL

The physician's order is for metronidazole (Flagyl®) 1.5 g PO tid x 7 days. The drug available is Flagyl 250 mg per tablet.

Step 1: Convert grams to milligrams: 1.5 g = 1500 mg (multiply the mass value (1.5) by 1000) Step 2: H/V = D/X 250 mg /1 tab = 1500 / X 1500/250 = 6 tabs per dose x 3 per day = 18 tabs per day for 7 days

Explain the difference between Arabic numbers and Roman numerals.

The Arabic system is commonly used for counting. Roman numerals uses letters to represent number values, but has no symbol for zero. Roman numerals are used less commonly than Arabic numbers in dosage calculation, although some physicians may still use this system in prescriptions. The pharmacy technician needs to understand Roman numerals to interpret physicians' orders and drug dosages precisely. In pharmacy settings, the technician usually sees and uses only the Roman numerals that represent the values 1 through 30. Roman numerals can be written as either uppercase or lowercase letters. In medical usage, "iv," which represents the number 4, is generally written in lowercase to differentiate it from "IV," which is the abbreviation for intravenous. The building blocks of the Roman system include the letters I, V, X, L, C, D, and M. Only I, V, and X are required to show the values 1 through 30. Most of the medications administered or ordered should be measured by amounts expressed in Arabic numbers. The familiar system of whole numbers (0 through 9), fractions (e.g., 1/5), and decimals (e.g., 0.7) is used widely in the United States and internationally.

Amoxicillin 500 mg PO qid (by mouth 4 times a day)(dose desired) is prescribed by the physician; the dose on hand is 250 mg/5 mL.

The formula is as follows: (H) 250 mg / (V) 5 mL = (D) 500 mg / X (dose desired) So, 5/1 x 500/250 = 2500 2500/250 = 10 mL (1 dose) (40 mL for the daily dosage)

Ratio and proportion is the oldest method used for calculating dosage. It is often referred to as means multiplied by extremes. One formula uses ratios based on the dose on hand and the dose desired. (see pg 162)

The formula is as follows: H : V :: D : X V and D are called the means H and X are called the extremes. Multiply the means and then the extremes and cross-multiply to solve for X.

Methods of Calculation

There are four methods for drug dosage calculations: basic formula, ratio and proportion, fractional equation, and dimensional analysis. The ratio and proportion and fractional equation methods are similar. When body weight and body surface area calculations are used, one of the first four methods for calculation is necessary to determine the amount of drug needed from the container. The pharmacist or technician commonly uses these methods when the calculation of drug doses is required.

A quick reference guide that calculates the body surface area (BSA) of infants and young children based on their height and weight.

West's nomogram

Dimensional analysis allows multiple ratios to be entered into a single equation. It is sometimes easier to do the conversion before the equation is set up. For instance, if a medication is available in milligrams (mg) but is labeled in another unit, such as grams or micrograms, metric conversion must be done. The IM dosage ordered is 250 mg. The drug available is labeled 1 g per 2 mL. How many mL must you give?

convert 1 g into 2 mL ... (1 g = 1000 mg) therefore mL = 2 mL/1000 mg (then) x by the dosage 250 mg/1 cancel out the mg's multiply across mL = 500/1000 answer is 0.5 or 1/2 mL

Because 1 L is equal to 1000 mL, how many fluidounces (fl oz) are in 2.5 L? Approximately 30 mL are in 1 fl oz. Use ratio and proportion as follows:

convert 2.5 L into mL (1000 mL x 2.5 L = 2500 mL) So, 1 fl oz / X fl oz = 30 mL / 2500 mL cross multiply = X30 = 2500 x 1 (divide both sides by 30) X = 2500/30 = 83.3 fl oz

An intramuscular medication order is for 0.5 mg of medication in solution. The drug label reads "750 mcg in 2 mL."

convert mcg to mg 1 mg = 1000 mcg mL = 2 mL / 750 mcg x 1000 mcg / 1 mg x 0.5 mg / 1 cancel out the mg and mcg and multiply across 2 x 1000 x 0.5 = 1000 750 x 1 x 1 = 750 1000/750 = 1.33 mL To give a dosage of 0.5 mg from the available 2 mL per 750 mcg strength, you must prepare 1.33 mL.

A numerator that is expressed in numerals with a decimal point placed so that it designates the value of the denominator, and the denominator, which is understood to be 10 or some power of 10; also called decimal fraction.

decimal

A fractional number with a denominator of 10 or a power of 10. Usually written with a decimal point.

decimal fraction

The bottom number in a fraction. Shows how many equal parts the item is divided into.

denominator

the number of drops an intravenous infusion is administered at over a specific period of time

drop rate

the form of the drug dose with its equivalent in units

drug label factor

the desired dose, dose on hand, and vehicle set up in an equation that helps to cancel the units to give the right answer in the right units for delivery

drug order factor

The two outside terms (first & last) in a proportion.

extremes

State the purpose of using West's nomogram.

is a quick reference guide that calculates the body surface area (BSA) of infants and young children based on their height and weight. The nomogram calculates BSA in square meters . To determine the BSA using West's nomogram, place a dot on the nomogram at the point that corresponds to the patient's height and another dot at the point that corresponds to the patient's weight; then draw a straight line between the two dots. The point where the line crosses the square meter column is the patient's body surface area. Be careful to place the dots correctly, as marking the scales incorrectly will result in calculation of an erroneous BSA. Many physicians use the nomogram as a quick reference in determining body surface area for pediatric doses.

List the three formulas used to calculate dosages for infants and children. 1. Clark's Rule

is based on the weight of the child This system is much more accurate than other pediatric methods, because the size and body weight of children of any age can vary greatly. Clark's rule uses 150 lb (70 kg) as the average adult weight and assumes that the child's dose is proportionately less. pediatric dose = Child's weight in lbs/150 lbs x Adult dose (= 100 mg) example: Find the dose of cortisone for a 30-lb infant. The calculation is as follows: 30/150 x 100 mg = 20 mg 30/150 = .2 x 100 = 20 mg

Basic Formula

is often used to calculate drug dosage. The basic formula that is most commonly used is: D/H x V = amount to give In this equation, ~ D stands for desired dose: the drug dose ordered by the prescriber. ~ H stands for on-hand dose: the drug dose on the label of the container (ampule, vial, or bottle). ~ V stands for vehicle: the form and amount in which the drug is supplied (capsule, tablet, or liquid).

2. Young's rule

is used for children older than 1 year. Note that Young's rule is not valid after 12 years of age. If the child is small enough to warrant a reduced dose after 12 years of age, the reduction should be calculated on the basis of Clark's rule. pediatric dose = Child's age in yrs/Child's age in yrs + 12 x Adult dose (1000 mg) example: Find the dose of acetaminophen for a 4-year-old child . The calculation is as follows: 4 / 4+12 x 1000 mg 4/16 = 1/4 or .25 x 1000 = 250 mg

The curved upper surface of a liquid in a tube

meniscus

Example: 2 1/2 A whole number and a proper fraction that are combined. The value of the mixed number is always greater than 1.

mixed fraction

The number written above the line in a fraction. It tells how many equal parts are described in the fraction.

numerator

A fraction whose numerator is expressed and whose denominator is understood to be 100.

percent

A fraction in which the numerator is smaller than the denominator and designates less than one whole unit.

proper fraction

The relationship between two equal ratios.

proportion

A mathematical expression that compares two numbers by division.

ratio

A prescription is written to give 100 mg of ampicillin with a dosage strength of 125 mg/5 mL. The following formula is used:

(D) = 100 mg / (H) = 125 mg x (V) = 5 mL 100 / 125 (25 goes into each) 4/5 4/5 x 5/1 multiply across = 20 / 5 ml = 4 mL

The physician's order is for Lopressor® 100 mg PO bid. The drug available is Lopressor 50 mg/tablet.

No conversion is necessary, patient must take 2 tablets by mouth twice a day.

To set up an equation for dimensional analysis, you must follow these steps:

1. Write the unit of measure being calculated, such as milliliters (mL). This eliminates confusion over which measure is being calculated and determines how the first clinical ratio is entered into the equation. 2. Identify the complete clinical ratio that contains the milliliters, as provided by the dosage strength available. This should be entered as a common fraction. 3. All additional ratios are entered so that each denominator matches with its successive numerator. If the first ratio denominator is milligrams, then the next numerator also must be in milligrams. 4. Cancel the alternate denominator/numerator measurement units (but not their quantities). They must match. If so, all clinical ratios were entered correctly. After cancellation, only the unit of measure being calculated may remain in the equation. 5. Calculate the equation now that all components are in place.

There are two types of fractions:

1. common fractions: A common fraction usually represents equal parts of a whole. It consists of two numbers and a fraction bar and is shown in the form: Numerator/Denominator example: 1/4, 2/5, and 9/10 are all common fractions. There are four types of common fractions: proper, improper, mixed, and complex. 2. decimal fractions: A decimal fraction is commonly referred to simply as a decimal (e.g., 0.5).

The physician's order is for Motrin® 600 mg PO bid. The drug available is Motrin 300 mg tablets.

300 mg / 1 tab = 600 mg / X X = 600/300 = 2 tabs

(proportion) A dosage of 30 mg has been ordered. The strength available is 35 mg in 2.5 mL.

35 mg/2.5 mL = 30 mg/X mL X = 2.5 x 30 / 35 (aka 75/35) = 2.1 (rounded up) Because the dosage ordered (30 mg) is smaller than the strength available (35 mg in 2.5 mL), the answer should be smaller than 2.5 mL, and it is.

What is a fraction?

A fraction is an expression of division with a number that is the portion or part of a whole amount. A fraction has two parts (these fractions are known as common fractions). The bottom is referred to as the denominator, which represents the whole. It can never equal zero. The numerator is the top part of the fraction and represents parts of the whole. 4 (Numerator) 6 (Denominator) reduce to 2/3

proportions

A proportion shows the relationship between two equal ratios. A proportion may be expressed as: 4 : 8 :: 1 : 2 or 4 : 8 = 1 : 2 where in the first case (::) is read as "so as." Thus, 4 is to 8 so as 1 is to 2. ?????? These ratios are equal because multiplying 1 and 2 by 4 will result in 4 and 8, respectively. In a proportion, the terms have names. 4 : 8 :: 1 : 2 The extremes are the two outside terms (4 and 2) and the means are the two inside terms (8 and 1). In a proportion, the product of the means is equal to the product of the extremes. An equation stating that two ratios are equal.

ratios

A ratio is a mathematical expression that compares two numbers by division. It is used to indicate the relationship of one part of a quantity to the whole. When written, the two quantities are separated by a colon (:). The use of the colon is a traditional way to write the division sign within a ratio, which is expressed as "3 is to 7." 3/7 may be expressed as a ratio 3:7 1:150 may be expressed as a fraction 1/150

Define "dimensional analysis."

Dimensional analysis is a method that is used for solving complicated pharmaceutical calculations. The dimensional analysis method (the label factor method) is used for calculating dosages with three factors, which include the following: Drug label factor The form of the drug dose (V) with its equivalence in unit (H). example: 1 tab = 250 mg Conversion factor Memorize the following factor conversions: Metric Equivalents 1 kg = 1000 g 1 g = 1000 mg 1 mg = 10000 mcg Metric Apothecary Equivalents 1 g = 15 gr 1000 mg = 15 gr 1 gr = 60 mg Drug order factor The three factors D, H, and V are set up in an equation that helps to cancel the units, giving the right answer in the right units for delivery. V = V (vehicle) x C (H) x D (desired)/ H (on hand) x 1

Explain the formula used to calculate liquid drugs.

For medications in liquid form, calculate the volume of the liquid that contains the ordered dosage of the medication. The label of bottled drugs may indicate the amount of drug per 1 mL or per multiple milliliters of the solution, for example, 20 mg/5 mL, 250 mg/5 mL, or 1.4 g/30 mL. Liquid drugs must be calculated with the formula: D/H x V = X In this formula, D represents the desired dosage or the dosage ordered. H represents the dosage you have on hand per a quantity. V represents the volume of the drug.

Fractional equation method is similar to the ratio and proportion method, except that the calculation is written as a fraction.

H/V = D/X H stands for the dosage on hand, V stands for vehicle, D stands for desired dosage, and X stands for the unknown amount to give. Cross-multiply and then solve for X.

LEFT side of decimal -- hundred thousands -- ten thousands -- thousands -- hundreds -- tens -- ones (or units)

RIGHT side of decimal -- tenths -- hundredths -- thousandths -- ten thousandths -- hundred thousandths

percentages

The term percent and its symbol, %, mean hundredths. A percentage is a fraction whose numerator is expressed and whose denominator is understood to be 100. It may be written in the form of "x" with the symbol %, or "x%." It can be changed to a decimal by moving the decimal point two places to the left to signify hundredths or to a fraction by expressing the denominator as 100. For example, the ratio of 30 to 100 is 30 percent (30%). This means the same thing as "30 hundredths," or 0.30. Percentages may be expressed as fractions, decimals, or ratios. All of these allow percentages to be expressed as "parts of a whole."

For calculations using dimensional analysis:

The unit of measure being calculated is written first to the left of the equation. The unit of measure is followed by an equal sign. All ratios entered must include the quantity and the unit of measure. The numerator in the starting ratio must be in the same measurement unit as the unit of measure being calculated. The unit of measure in each denominator must be matched in the numerator of each successive ratio entered. Incorporating a conversion ratio directly into the dimensional analysis equation can make metric system conversions. The unit of measure in each alternate denominator and numerator must cancel. This leaves only the unit of measure being calculated remaining in the equation. The numerator of the starting ratio is never cancelled.

Change an improper fraction to a mixed fraction.

To convert an improper fraction to a mixed fraction, follow the rule: Divide the numerator by the denominator. The result will be a whole number plus a remainder. The remainder is the numerator over the original denominator. Mix the whole number and the fractional remainder. This mixed fraction equals the original improper fraction.

Abbreviation Meanings

ac ~ before meals bid ~ twice a day cap ~ capsule ID ~ intradermally (within the skin) IM ~ intramuscularly (into the muscle) IV ~ intravenously (into the vein) SC ~ subcutaneously (in fatty tissue or under the skin) gt ~ drop hs ~ at bed time od ~ right eye os ~ left eye po ~ by mouth pc ~ after meals pil ~ pill prn ~ as needed q2h ~ every 2 hours qd ~ every day qh ~ every hour qid ~ 4 times a day tab ~ tablet tid ~ 3 times a day

For reading and writing decimals

all whole numbers are to the left of the decimal point and all fractions are to the right. See page 154 table 9-2 for decimal line Always write a zero to the left of the decimal point when a decimal has no whole number "part." Using the zero makes the decimal point more noticeable and helps to prevent errors caused by illegible handwriting.

Represents equal parts of a whole. Any fraction whose numerator and denominator is a common multiple of the denominators.

common fraction

Used to determine equivalents of specific units of measure. A ratio derived from the equality between two different units that can be used to convert from one unit to the other

conversion factor

To solve the same equation by dimensional analysis, you would do the following:

fl oz = 1 fl oz/30 mL x 1000 mL/1 L (cancel out the mL and multiply across) fl oz = 1000/30 = 33.3 x 2.5 = 83.25 and round up to 83.3 fl oz

An expression of division with a number that is the portion or part of a whole.

fraction

improper fraction

has a numerator that is greater than or equal to the denominator. The value of an improper fraction is greater than (>) or equal (=) to 1 6/4 = > 1 Whenever the numerator and denominator are equal, the value of the improper fraction is always equal to 1 6/6 = 1

proper fraction

has a numerator that is smaller than the denominator and designates less than one whole unit. Whenever the numerator is less than the denominator, the value of the fraction must be less (<) than 1 3/5 = < 1

mixed fraction

has a whole number and a proper fraction that are combined. The value of the mixed fraction is always greater (>) than 1 1 3/5 = 1 + 3/5 = >1

A fraction whose numerator is greater than or equal to its denominator.

improper fraction

3. Fried's rule

is a method of estimating the dose of medication for a pediatric patient younger than 24 months when given only the adult dose. This method should not be considered as accurate as the nomogram method because it is based on the assumption that the child is of average size and utilizes age rather than weight. It is important to note that because age does not necessarily indicate the patient's weight, medication adjustments may be necessary once the patient's response is determined. pediatric dose = Child's age in months/ 150 x Average Adult dose Find the dose of phenobarbital for a 15-month-old infant. (adult dose in this case is 400 mg) The calculation is as follows: 15 / 150 x 400 = 40 mg

the middle terms of a proportion

means