brain teasers, BrainTeasers, Brain Teasers, Brain Teasers - Interview Questions

Using only addition, how do you add eight 8's and get the number 1000?

888 + 88 + 8 + 8 + 8 = 1000

What is the angle between the hour-hand and minute-hand of a clock at 3:15?

At quarter past the hour, the minute-hand is exactly at 3:00 but the hour-hand has moved 1/4 of the way between 3:00 and 4:00. Therefore 1/4 times 1/12 = 1/48 of the clock. With the clock having 360 degrees, 360/48 = 7.5 degrees.

You've got a 10 x 10 x 10 cube made up of 1 x 1 x 1 smaller cubes. The outside of the larger cube is completely painted red. On how many of the smaller cubes is there any red paint?

First, note that the larger cube is made up of 1000 smaller cubes. The easiest way to think about this is how many cubes are NOT painted? 8 x 8 x 8 inner cubes are not painted which equals 512 cubes. Therefore, 1000 - 512 = 488 cubes that have some paint.

How many NYSE-listed companies have 1 letter ticker symbols?

It could be 26 (letters in the English alphabet), but it is actually only 24 because I & M are saved for Intel and Microsoft, in case they change their minds.

How many street lights are there in NYC?

Let's start by estimating lights on the ordered grid of Manhattan. Estimate the number of horizontal blocks and vertical blocks, and then the number of street lights on each. Then multiply by 5 to account for the other 5 boroughs.

You have 20 bottles of pills. 19 bottles have 1.0 gram pills, but one has pills of weight 1.1 grams. Given a scale that provides an exact measurement, how would you find the heavy bottle?

Take 1 pill from bottle 1, 2 pills from 2 and 3 pills from 3. Using this we can derive a formula that allow us to figure out which the bottle. (weight-210 grams) / 0.1 grams. or weight - (weight if normal) / 0.1 grams

What source gives natural rubber

Trees

If 1 1/2 sardines cost 1 1/2 dollars how much would 7 1/2 sardines cost

$7.50

Which 3 numbers have the same answer whether they're added or multiplied together?

1, 2 and 3.

There is a three digit number. The second digit is four times as big as the third digit, while the first digit is three less than the second digit. What is the number?

141

Sally is 54 years old and her mother is 80, how many years ago was Sally's mother three times her age?

41 years ago, when Sally was 13 and her mother was 39.

A scientist puts a bacteria in a petri dish at exactly noon. Every minute, the bacteria divides into two. At exactly 1 pm, the petri dish is full. At what time was the dish half full?

If the bacteria doubles every minute, and it's full at 1 pm, it must have been half-full a minute earlier, at 12:59.

I am an odd number. Take away one letter and I become even. What number am I?

Seven (take away the 's' and it becomes 'even').

what is so fragile that when you say its name you break it.

Silence

A man gets up 180 times every night and sleeps for at least 7 hours at a stretch. Where does he live?

north or south pole

How many times can you subtract 6 from 30

one time

What occurs once in June and twice in August but never in October?

the letter U

Yesterday afternoon I went to visit my friend albert the painter-while I watched him paint I said" No wonder it takes so long for you to finish since I have been here you have entered the studio 12 times- how many times did Albert leave the room

11 times(one less than the times he came in)

Given 2 ropes, each takes 1 hour to burn. How would you use them to time exactly 15 minutes? The rope has uneven densities and therefore half the rope length wise does not necessary take half an hour to burn.

1) Burn 1st rope on both end 2) Burn 2nd rope on one end 3) When rope 1 completed then burn rope 2 on the other end.

You have a 100 coins laying flat on a table, each with a head side and a tail side. 10 of them are heads up, 90 are tails up. You can't feel, see or in any other way find out which side is up. Split the coins into two piles such that there are the same number of heads in each pile.

ick any 10 coins and make a small pile. You now have 90 coins in a pile (call it pile A), and 10 coins in a new pile (call it pile B). You don't know where the heads are, but you do know that there are 10 of them in total, and therefore however many heads there are in pile A, there is 10 minus this many in pile B. Let's do an inventory: Pile A contains: - n heads - 90-n tails Pile B contains: - 10-n heads - the rest tails. How many tails exactly? Well out of 10 coins, 10-n are heads, which means the number of tails is 10-(10-n) = n tails. Wait...what? pile A has n heads, and pile B has n tails? So we know that there are always the same number of heads in pile A as there are tails in pile B, even if we don't know exactly what number that is? So flip over all the coins in pile B. We now have the same number of heads in both piles.

The butcher, his daughter, the fisherman and his wife won the lottery and divided the prize equally into three- how can this be.

the fisherman wife is the butcher's daughter

A street that's 30 yards long has a chestnut tree every 6 yards on both sides- how many chestnut trees on the entire street

twelve

I climbed up a cherry tree, where I found cherries, I did not pick cherries, nor did I leave cherries. How can you explain this?

two cherries were all that were there

What is the smallest and least powerful piece on a chessboard

Pawn

What is the angle formed by the hands of the clock when it is 1:45?

142.5 degrees. If we just think of the clock hour hand at 1 and the minute hand at the 45 position (near 9 o'clock), that is 120 degrees since they are 4 "numbers" apart, and each number on the clock represents 30 degrees (360/12). However, recall that the hour hand has already moved by the time the minute hand has reached the 45 position - it is now closer to 2 o'clock. 45 represent 3⁄4 of an hour, so the hour hand will have moved 3⁄4 of 30 degrees, or 22.5 degrees. If we add them together, we see that 120 + 22.5 = 142.5 No formula, think about it

There is a basket containing 5 apples, how do you divide the apples among 5 children so that each child has 1 apple while 1 apple remains in the basket?

4 children get 1 apple each while the fifth child gets the basket with the remaining apple still in it.

A friend was telling me" I have eight sons and each has one sister". In total how many children does my friend have?

9 total children

How many quarters would it take to make a stack from the floor of this room to the ceiling?

Assume the room is 10 feet high, and assume that 12 quarters is 1 inch tall ... Answer: Assume the room is 10 feet high, and assume that 12 quarters is 1 inch tall ... Osvaldo Zoom 12 quarters / inch x 12 inches / feet x 10 feet room = 1440 quarters

"How many barbers are there in Chicago?"

Assume x number of people Say half are male Say 1 in every 100 are barbers Arrive at conclusion

Two boats are going towards each other at 10 miles per hour. They are 5 miles apart. How long until they hit?

Be careful here. The initial instinct is to say half an hour. However, both boats are moving at 10 miles per hour, so they are converging at 20 miles per hour, meaning they will crash in 1⁄4 of an hour, or 15 minutes. [Divide the miles by the Speed to get the time]

Two girls were born to the same mother, at the same time, on the same day, in the same month and in the same year and yet somehow they're not twins. Why not?

Because there was a third girl, which makes them triplets!

Given 9 balls, tell me which ball is the heaviest after only 2 tries using a scale.

Divide the group into 3 and weight each group. After the 1st weight, we know which group has the heavy ball

There is a building with 100 floors. If an egg drops from the Nth floor or above it, it will break. If it's dropped from any floor below , it will not break. You're given 2 eggs. Find N, while minimizing the number of drops for the worst case.

Don't think that the egg will necessary break. Assuming that the egg is surrounded by some kind of material. The answer is really about minimizing the number of steps regardless. The answer is start at floor 14, 27, 39 for a total of 14 steps in the worse case. https://www.quora.com/What-is-the-solution-to-the-dropping-eggs-puzzle

What is the probability that the first business day of a month is a Monday?

Each day has a 1 in 7 chance of being the first day of the month. However, if the month starts on a Saturday or a Sunday, the first business day of the month will be a Monday. Therefore, the chances of the first business day being a Monday is 3 in 7 since if the month starts on Saturday, Sunday or Monday, the first business day is a Monday.

How much does the Starbucks in Times Square bring in, in annual revenue?

Even for an insanely busy Starbucks, you can get pretty far by estimating the average ticket cost and the average traffic per hour, multiplied by 24, multiplied by 365. As always with market sizing questions, justify your assumptions, and use round numbers so you can get an estimate quickly and without errors.

You have 5 quart jug and a 3 quart jug and an unlimited supply of water. How do you come up with exactly 4 quarts of water. Jugs are oddly shaped, so filling up exactly half is not possible.

Fill both jar at the same time. When the 3 quart jug is filled, stop the water. Put the water in 3 quart jug into 5 quart jug. That means only 1 quart is left in the 3 quart jug. Place the 1 quart water into another jar. Fill the 3 quart jar and that's how you get to 4 quart of water. Alternative answer 5 0 2 3 2 0 0 2 5 2 4 3 4

You are given a 3-gallon jug and a 5-gallon jug. How do you use them to get 4 gallons of liquid?

Fill the 5-gallon jug completely. Pour the contents of the 5-gallon jug into the 3-gallon jug, leaving 2 gallons of liquid in the 5-gallon jug. Next, dump out the contents of the 3-gallon jug and pour the contents of the 5-gallon jug into the 3-gallon jug. At this point, there are 2 gallons in the 3-gallon jug. Fill up the 5-gallon jug and then pour the contents of the 5-gallon jug into the 3-gallon jug until the 3-gallon jug is full. You will have poured 1 gallon, leaving 4 gallons in the 5-gallon jug.

Four investment bankers need to cross a bridge at night to get to a meeting. They have only one flashlight and 17 minutes to get there. The bridge must be crossed with the flashlight and can only support two bankers at a time. The Analyst can cross in 1 minute, the Associate can cross in 2 minutes, the VP can cross in 5 minutes and the MD takes 10 minutes to cross. How can they all make it to the meeting in time?

First, the Analyst takes the flashlight and crosses the bridge with the Associate. This takes 2 minutes. The Analyst then returns across the bridge with the flashlight taking 1 more minute (3 minutes passed so far). The Analyst gives the flashlight to the VP and the VP and MD cross together taking 10 minutes (13 minutes passed so far). The VP gives the flashlight to the Associate, who recrosses the bridge taking 2 minutes (15 minutes passed so far). The Analyst and Associate now cross the bridge together taking 2 more minutes. Now, all are across the bridge at the meeting in exactly 17 minutes. Note, that instead of investment bankers, you'll often see the same question using members of musical bands (usually either the Beatles or U2).

You are given 12 balls and a scale. Of the 12 balls, 11 are identical and 1 weighs slightly more. How do you find the heavier ball using the scale only three times?

First, weigh 5 balls against 5 balls (1st Use of Scale). If the scale is equal, then discard those 10 balls and weigh the remaining 2 balls against each other (Second Use of Scale). The heavier ball is the one you are looking for. If on the first weighing (5 vs 5), one group is heavier, then of the heavier group weigh 2 against 2 (2nd Use of Scale). If they are equal, then the 5th ball from the heavier group (the one not weighed) is the one you are looking for. If one of the groups of 2 balls is heaver, then take the heaver group of 2 balls and weigh them against each other (Third Use of Scale). The heavier ball is the one you are looking for.

A car travels a distance of 60 miles at an average speed of 30 mph. How fast would the car have to travel the same 60 mile distance home to average 60 mph over the entire trip?

Most people say 90 mph but this is actually a trick question! The first leg of the trip covers 60 miles at an average speed of 30 mph. So, this means the car traveled for 2 hours (60/30). In order for the car to average 60 mph over 120 miles, it would have to travel for exactly 2 hours (120/60). Since the car has already traveled for 2 hours, it is impossible for it to average 60 mph over the entire trip.

You have 100 balls (50 black balls and 50 white balls) and 2 buckets. How do you divide the balls into the two buckets so as to maximize the probability of selecting a black ball if 1 ball is chosen from 1 of the buckets at random?

Just to be perfectly clear, you are assuming that one of the two buckets is chosen at random and then one of the balls from that bucket is chosen at random. You want to put 1 black ball in 1 of the buckets and all of the other 99 balls in the other bucket. This gives you just slightly less than a 75% change of having a black ball chosen. The math works as follows: There's a 50% chance of selecting the bucket containing 1 ball with a 100% chance of selecting a black ball from that bucket. And a 50% chance of selecting the bucket containing 99 balls with a ~49.5% (49/99) chance of selecting a black ball from that bucket. Total probability of selecting a black ball is (50% % 100%) + (50% * 49.5%) = 74.7%.

Why Are Manhole Covers Round?

Manhole covers are round so that they won't fall into the manholes. Some other answers: • Because manholes are round • Because they are easier to transport (just roll them) • Because they are easier/cheaper to manufacture (smaller surface area than a square cover)

If you were going to build a building in a city and had no physical restraints, no capital restraints, and no other limitations, how tall would you build it?

Measuring the demand for space in a new building How high people would be willing to purchase space due to safety concerns How much you can sell the space for in comparison with how much it costs to construct/maintain How much the demand for the space will grow over the life of the building, so how much extra space should you build into the design

My friend Albert the butcher wears a size 13 shoe, is six feet tall, and wears a 42 suit- what does he weigh?

Meat

What is the probability of five people with different ages sitting in ascending or descending order at a round table?

Number people 1, 2 , 3 ,4 , 5 meaning their ordinal age; there are five ways they can sit around a round table in increasing order of age: [ 1 2 3 4 5 ] [2 3 4 5 1] etc. There are 5 x 4 x 3 x 2 x 1 possible seating arrangements around the table, so dividing 5 by 5 x 4 x 3 x 2 we get probability = 1/24

What word looks the same backwards and upside down?

SWIMS

What is the greatest dollar value in coins you can have in your hand and still not be able to make change for a dollar?

Start with quarters since they have the highest value. Then descend in value from dimes to nickels to pennies, taking the maximum number of each that you can have without reaching a dollar. The maximum you can take will be 3 quarters, 4 dimes, 0 nickels and 4 pennies. Remember that you can't take a nickel because 3 quarters, 2 dimes and a nickel would add up to a dollar. The total value of these coins is $1.19.

How would you test a calculator?

Test the functionality of the calculator's computing accuracy by evaluating whether inputs provide their expected outputs. Also, test the device's basic system functions — like the power button, the clear function, and so on, to determine whether you'll be facing system errors before you can compute anything.

Three envelopes are presented in front of you by an interviewer. One contains a job offer, the other two contain rejection letters. You pick one of the envelopes. The interviewer then shows you the contents of one of the other envelopes, which is a rejection letter. The interviewer now gives you the opportunity to switch envelope choices. Should you switch?

The answer is yes. Say your original pick was envelope A. Originally, you had a 1/3 chance that envelope A contained the offer letter. There was a 2/3 chance that the offer letter was either in envelope B or C. If you stick with envelope A, you still have the same 1/3 chance. Now, the interviewer eliminated one of the envelopes (say, envelope B), which contained a rejection letter. So, by switching to envelope C, you now have a 2/3 chance of getting the offer and you've doubled your chances.

A doctor has a brother who is an attorney in Alabama, but the attorney in Alabama does not have a brother who is a doctor how can this be?

The doctor is a woman

How many gas stations are in the United States?

The easiest way to go about answering a question like this is to start small and work your way out. Think about your town. Say your town has 30,000 people, and you have 5 gas stations serving that area. The United States has approximately 300 million people, so that means there are 10,000 "towns" in the United States, and 50,000 gas stations. You then want to make adjustments. For example, assume that a quarter of the population lives in larger cities where there is only 1 gas station per 30,000 people. So you have 7,500 towns with 5 gas stations and 2,500 "towns" with only 1. Do a little mental math and you get a number of 40,000 gas stations in the U.S.

You are late for a pitch with the CEO of a company in the Town of Truth. You are speeding down a road that suddenly forks, and there are no signs. You know that one way leads to the Town of Truth where everyone tells the truth and the other way leads to the Town of Lies where everyone tells lies. There is a resident of one of the towns standing at the crossroads but you don't know which town he's from. You only have time to ask him one question.... So what do you ask him?

The key to this question is to ask a question which both guys would have to answer the same way. If you ask him to point out which town he is from, either of them would point to the Town of Truth.

A ship anchored in a port has a ladder which hangs over the side. The length of the ladder is 200cm, the distance between each rung in 20cm and the bottom rung touches the water. The tide rises at a rate of 10cm an hour. When will the water reach the fifth rung?

The tide raises both the water and the boat so the water will never reach the fifth rung.

What is the sum of numbers from 1 to 100?

The trick here is that you have 50 pairs which each sum to 101 (e.g. 1+100, 2+99, 3+98, etc.). So, 50 times 101 = 5050.

A stock is trading at 10 and 1/16. There are 1 million shares outstanding. What is the stock's Market Cap?

This is just a test of your mental math. If a fourth is .25, an eighth is .125, and a sixteenth is .0625... The stock price is 10.0625 and the Market Cap is 10.0625 million.

A windowless room has 3 light bulbs. You are outside the room with 3 switches, each controlling one of the lightbulbs. If you can only enter the room one time, how can you determine which switch controls which light bulb?

Turn on two switches (call them A and B) on and leave them on for a few minutes. Then turn one of them off (switch B) and enter the room. The bulb that is lit is controlled by switch A. Touch the other two bulbs (they should be off). The one that is still warm is controlled by switch B. The third bulb (off and cold) is controlled by switch C.

Every day, a cyclist crosses the border between Spain and France carrying a bag. No matter how much customs officials investigate him, they do not know what he is smuggling- do you?

bicycles

A mother has six children and five potatoes. How can she feed an equal amount of potatoes to each child with out using fractions

boil and mash the potatoes