Math Lesson 57 Adding and Subtracting with Unlike Denominators

If 1/3 is equivalent to 2/6 then what is the sum??

1/3 + 2/6 = 2/6 + 2/6 = 4/6

least common denominator (LCD)

The least common multiple of two or more denominators.

Add, write the answer in simplest form. 1/16 + 9/16 a. 10/16 b. 5/8 c. 1/2 d. 3/4

b. 5/8

What is 3/4 minus 1/8? a. 1/4 b. 5/8 c. 1/2 d. 1/3

b. 5/8

Add or subtract. Write in simplest form. 2/3 - 3/5 a. -1/8 b. 1 4/15 c. 1/15 d. 2/5

c. 1/15

Add or subtract. Write in simplest form. 1/9 + 1/8 a. 1/36 b. 1/72 c. 17/72 d. 2/17

c. 17/72

Add or subtract. Write in simplest form. 2/5 - 1/4 a. 1/9 b. 13/20 c. 3/20 d. 1/10

c. 3/20

How much longer than 9/16 inch is 7/8 inch? a. 2/8 inch b. 1/4 inch c. 5/16 inch d. 1/8 inch

c. 5/16 inch

Find the sum: 1/3 + 1/4 a. 1/12 b. 2/7 c. 7/12 d. 1

c. 7/12

Find the difference: 4/5 - 1/3 a. 3/2 b. 3/15 c. 7/15 d. 17/15

c. 7/15

Add or subtract. Write in simplest form 5/8 - 1/3 a. 23/24 b. 5/24 c. 7/24 d. 4/11

c. 7/24

How much more is 3/4 ounce than 1/3 ounce? a. 1/2 ounce b. 4/7 ounce c. 1/6 ounce d. 5/12 ounce

d. 5/12 ounce

Add or subtract. Write in simplest form. 1/6 + 1/8 a. 1/12 b. 1/48 c. 1/7 d. 7/24

d. 7/24

Name the three steps needed to add or subtract fractions with unlike denominators. use the words first, next, and last.

First you find the least common multiple of the denominator. Then you keep the denominator the same and do what you did to the bottom to the top. Lastly you just add or subtract to get your answer. 3/4 - 1/6 = 4: 4, 8, 12, 16, 20 6: 6, 12, 18, 24, 30 3/12 - 1/12 9/12 - 2/12 = 7/12

Adding and Subtracting Fractions with Unlike Denominators

In this lesson, we will learn how to add and subtract fractions with unlike denominators. To find the sum or difference of two fractions with unlike denominators, rename the fractions using the least common denominator (LCD). Then add or subtract the numerators only and simplify.

Common Error Alert!!

Many students mistakenly add or subtract both the numerator and the denominator. Make sure you keep the denominator the same and only add or subtract the numerator!!

Seki: because you don't add both the top and the bottom numbers. Instead you rename them using a common denominator after finding the LCM, then you add the numerator but the denominator stays the same.

Victor and Seki are finding 3/4 + 1/2. Who is correct?

b. Figure B

Which of the following fraction models represents the difference 4/5 - 2/3? a. Figure A b. Figure B c. Figure C d. Figure D

Add or subtract. Write in simplest form. 5/6 - 4/5 a. 1/30 b. 1/11 c. 1 19/30 d. 2/3

a. 1/30

Add or subtract. Write in simplest form. 5/12 - 1/4 a. 1/6 b. 2/3 c. 5/48 d. 1/4

a. 1/6

How much more is 3/8 gallon than 1/4 gallon? a. 1/8 gallon b. 1/4 gallon c. 1/2 gallon d. 1/3 gallon

a. 1/8 gallon

Estimate the sum or difference. 12 1/5 - 4/5 a. 11 b. 13 c. 12 d. 10

a. 11

Add or subtract. Write in simplest form. 3/5 + 2/7 a. 31/35 b. 5/12 c. 1/7 d. 6/35

a. 31/35

Subtract, and write in simplest form. 5 5/8 - 3/8 a. 5 1/4 b. 2 13/16 c. 6 d. 2 5/8

a. 5 1/4

Find the sum of 3/10 and 2/5 a. 7/10 b. 1/5 c. 5/15 d. 1/2

a. 7/10

Add or subtract. Write in simplest form 5/8 + 4/5 a. 1/2 b. 1 17/40 c. 9/13 d. 9/40

b. 1 17/40

Add or subtract. Write in simplest form. 4/15 - 1/5 a. 4/75 b. 1/15 c. 3/20 d. 7/15

b. 1/15

Which of the following is the lowest common denominator for the fractions 5/6 and 3/4? a. 4 b. 12 c. 15 d. 24

b. 12

Add or subtract. Write in simplest form. 1/9 + 5/6 a. 2/5 b. 17/18 c. 5/54 d. 1/3

b. 17/18