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A Collection of Quant Riddles With Answers

The quant riddles or logic or lateral puzzles 1-19 appear in the book 'Heard on The Street: Quantitative Questions from Wall Street Job Interviews' by Timothy Falcon Crack PhD available on his website, the rest have been accumulated from the internet and emails I receive. They are designed to help practice for job or university interviews or just training your brain. I've thought about them all and answered all but one. I've added a 'Helpful Hint' button to each question. I've tried to keep the clues appropriate to the difficulty of the question but don't feel you have to read the whole thing, they add ideas incrementally. Questions 3 & 5 are probably the easiest and a good place to start. I've coloured them Red, Amber and Green to indicate the difficulty along with the bar on the right. Difficulty is very subjective, but they all offer something in teaching you the ideas and tricks used to solve them. So that's it good luck....
Minimum Difficulty Maximum Difficulty
picture of an angry looking bug  3. You are a bug sitting in one corner of a cubic room. You wish to walk (no flying) to the extreme opposite corner (the one farthest from you). Describe the shortest path that you can walk.
 4. A mythical city contains 100,000 married couples but no children. Each family wishes to "continue the male line", but they do not wish to over-populate. So, each family has one baby per annum until the arrival of the first boy. For example, if (at some future date) a family has five children, then it must be either that they are all girls, and another child is planned, or that there are four girls and one boy, and no more children are planned. Assume that children are equally likely to be born male or female.

Let p(t) be the percentage of children that are male at the end of year t. How is this percentage expected to evolve through time?
picture of an clock  5. How many degrees (if any) are there in the angle between the hour and minute hands of a clock when the time is a quarter past three?

Person No. 1 enters the room, switches on every bulb, and exits. Person No. 2 enters and flips the switch on every second bulb (turning off bulbs 2, 4, 6...). Person No. 3 enters and flips the switch on every third bulb (changing the state on bulbs 3, 6, 9...). This continues until all 100 people have passed through the room.

 7. A windowless room contains three identical light fixtures, each containing an identical light bulb or light globe. Each light is connected to one of three switches outside of the room. Each bulb is switched off at present. You are outside the room, and the door is closed. You have one , and only one, opportunity to flip any of the external switches. After this, you can go into the room and look at the lights, but you may not touch the switches again. How can you tell which switch goes to which light?
 8. What is the smallest positive integer that leaves a remainder of 1 when divided by 2, remainder of 2 when divided by 3, a remainder of 3 when divided by 4, and so on up to a remainder of 9 when divided by 10?

The time comes, and a stranger arrives. He announces that there are cheating men in the town. On the morning of the 10th day following the stranger's arrival, some unfaithful men are kicked out into the street for the first time. How many of them are there?
picture of an old fashioned balance This puzzle is so hard, despite it being #1 I've moved it down the page.


 2. You are given a set of scales and 90 coins. The scales are of the same type as above. You must pay $100 every time you use the scales.

The 90 coins appear to be identical. In fact, 89 of them are identical, and one is of a different weight. Your task is to identify the unusual coin and to discard it while minimizing the maximum possible cost of weighing (another task might be to minimizing the expected cost of weighing). What is your algorithm to complete this task? What is the most it can cost to identify the unusual coin?
picture of some smiley facs playing the game  10. You and I are to play a competitive game. We shall take it in turns to call out integers. The first person to call out '50' wins. The rules are as follows:
–The player who starts must call out an integer between 1 and 10, inclusive;
–A new number called out must exceed the most recent number called by at least one and by no more than 10.

Do you want to go first, and if so, what is your strategy?
 11. You are to open a safe without knowing the combination. Beginning with the dial set at zero, the dial must be turned counter-clockwise to the first combination number, (then clockwise back to zero), and clockwise to the second combination number, (then counter-clockwise back to zero), and counter-clockwise again to the third and final number, where upon the door shall immediately spring open. There are 40 numbers on the dial, including the zero.

Without knowing the combination numbers, what is the maximum number of trials required to open the safe (one trial equals one attempt to dial a full three-number combination)?
 12. Inside of a dark closet are five hats: three blue and two red. Knowing this, three smart men go into the closet, and each selects a hat in the dark and places it unseen upon his head.

Once outside the closet, no man can see his own hat. The first man looks at the other two, thinks, and says, "I cannot tell what color my hat is." The second man hears this, looks at the other two, and says, "I cannot tell what color my hat is either." The third man is blind. The blind man says, "Well, I know what color my hat is." What color is his hat?
 13. You are standing at the centre of a circular field of radius R. The field has a low wire fence around it. Attached to the wire fence (and restricted to running around the perimeter) is a large, sharp-fanged, hungry dog. You can run at speed v, while the dog can run four times as fast. What is your running strategy to escape the field?
 14. You have 52 playing cards (26 red, 26 black). You draw cards one by one. A red card pays you a dollar. A black one fines you a dollar. You can stop any time you want. Cards are not returned to the deck after being drawn. What is the optimal stopping rule in terms of maximizing expected payoff?

Also, what is the expected payoff following this optimal rule?
 15. Why is it that if 'p' is a prime number bigger than 3, then p2-1 is always divisible by 24 with no remainder?
picture of a small chessboard  16. You have a chessboard (8x8) plus a big box of dominoes (each 2x1). I use a marker pen to put an "X" in the squares at coordinates (1, 1) and (8, 8) - a pair of diagonally opposing corners. Is it possible to cover the remaining 62 squares using the dominoes without any of them sticking out over the edge of the board and without any of them overlapping? You cannot let the dominoes stand on their ends.
 17. You have a string-like fuse that burns in exactly one minute. The fuse is inhomogeneous, and it may burn slowly at first, then quickly, then slowly, and so on. You have a box of matches, and no watch. How do you measure exactly 30 seconds?
If you had 2 fuses could you measure 45 seconds?
 18. Can the mean of any two consecutive prime numbers ever be prime?
 19. How many consecutive zeros are there at the end of 100! (100 factorial). How would your solution change if the problem were in base 5? How about in Binary???
how can this be true thumbnail  20. How can this be true???? Have a look at the picture (click to enlarge.) All the lines are straight, the shapes that make up the top picture are the same as the ones in the bottom picture so where does the gap come from????
 21. A man is in a rowing boat floating on a lake, in the boat he has a brick. He throws the brick over the side of the boat so as it lands in the water. The brick sinks quickly. The question is, as a result of this does the water level in the lake go up or down?
picture of a 3 and 5 litre water bottle under a tap  22. You have a 3 and a 5 litre water container, each container has no markings except for that which gives you it's total volume. You also have a running tap. You must use the containers and the tap in such away as to exactly measure out 4 litres of water. How is this done?
 23. I have three envelopes, into one of them I put a £20 note. I lay the envelopes out on a table in front of me and allow you to pick one envelope. You hold but do not open this envelope. I then take one of the envelopes from the table, demonstrate to you that it was empty, screw it up and throw it away. The question is would you rather stick with the envelope you have selected or exchange it for the one on the table. Why? What would be the expected value to you of the exchange?
 24. You're a farmer. You're going to a market to buy some animals. On the market there are 3 types of animals for sale. You can buy:
Horses for £10 each, goats for £1 each and ducks, you get 8 of these per bunch and each bunch costs £1. The aim is to acquire 100 animals at the cost of £100, what is the combination of horses, goats and duck that allows you to do this? (you must buy at least one of each.)


gas water electric puzzle layout  26. A man has built three houses. Nearby there are gas water and electric plants. The man wishes to connect all three houses to each of the gas, water and electricity supplies.

Unfortunately the pipes and cables must not cross each other. How would you connect each of the 3 houses to each of the gas, water and electricity supplies?
picture of a small chessboard  27. How many squares are there on a chessboard or checkerboard?? (the answer is not 64)

Can you extend your technique to calculate the number of rectangles on a chessboard?
 28. 3 men go into a hotel. The man behind the desk says a room is $30 so each man pays $10 and goes to the room.

A while later the man behind the desk realized the room was only $25 so he sent the bellboy to the 3 guys' room with $5. On the way the bellboy couldn't figure out how to split $5 evenly between 3 men, so he gave each man a $1 and kept the other $2 for himself.

This meant that the 3 men each paid $9 for the room, which is a total of $27 add the $2 that the bellboy kept = $29. Where is the other dollar?
 29. There were two men having a meal. The first man brought 5 loaves of bread, and the second brought 3. A third man, Ali, came and joined them. They together ate the whole 8 loaves. As he left Ali gave the men 8 coins as a thank you. The first man said that he would take 5 of the coins and give his partner 3, but the second man refused and asked for the half of the sum (i.e. 4 coins) as an equal division. The first one refused.

They went to Ali and asked for the fair solution. Ali told the second man, "I think it is better for you to accept your partner's offer." But the man refused and asked for justice. So Ali said, "then I say that who offered 5 loaves takes 7 coins, and who offered 3 loaves takes 1 coin."

Can you explain why this was actually fair?
 30. A drinks machine offers three selections - Tea, Coffee or Random but the machine has been wired up wrongly so that each button does not give what it claims. If each drink costs 50p, how much minimum money do you have to put into the machine to work out which button gives which selection ?
 31. Consider a standard chess board. What is the diameter of the largest circle that can be drawn on the board whilst only drawing on the black squares.
 32. Two creepers, (climbing plants,) one jasmin and other rose, are both climbing up and round a cylindrical tree trunk. jasmine twists clockwise and rose anticlockwise, both start at the same point on the ground. before they reach the first branch of the tree the jasmine had made 5 complete twists and the rose 3 twists. not counting the bottom and the top, how many times do they cross?
 33. Assuming I want to stay dry. Should i walk or run in the rain?
 34. A man has three daughters. A second, intelligent man, asked him the ages of his daughters. The first man told him that the product of their ages (them all multiplied together,) was 36. After thinking the second man was unable to find the answer and asked for another clue. The first man replies the sum of their ages is equal to his house door number. Still the second man was unable to answer and asked for another clue. The first man told him that his youngest daughter had blue eyes, and suddenly second man gave the correct answer. Explain how?
picture of a clock with overlapping hands  35. A regular clock has an hour and minute hand. At 12 midnight the hands are exactly aligned. When is the next time they will exactly align or overlap?
 36. You are in a dark room with a deck of cards. N of the cards are face up and the rest are face down. You can't see the cards. How do you divide the deck in to two piles with equal numbers of face up cards in each?
picture of an aero plane  37. If an aeroplane makes a round trip and a wind is blowing, is the trip time shorter, longer or the same?
 38. On a deserted road, the probability of observing a car during a thirty-minute period is 95%. What is the chance of observing a car in a ten-minute period?
two men in red and blue hats  39. An executioner lines up 100 prisoners single file and puts a red or a blue hat on each prisoner's head. Every prisoner can see the hats of the people in front of him in the line - but not his own hat, nor those of anyone behind him. The executioner starts at the end (back) and asks the last prisoner the colour of his hat. He must answer "red" or "blue." If he answers correctly, he is allowed to live. If he gives the wrong answer, he is killed instantly and silently. (While everyone hears the answer, no one knows whether an answer was right.) On the night before the line-up, the prisoners confer on strategy to help them. What should they do?
 40. Using only a 4 minute and 7 minute hourglass or egg timer how would you measure exactly 9 minutes?
 41. Suppose you tie a rope tightly around the Earth's equator. You add an extra 3 feet to the length. All around the Earth the rope is raised up uniformly as high as is possible to make it tight again. How high is that?
 42. You are at a point on the Earth. You travel South 100 miles, East 100 Miles and North 100 Miles. You arrive back where you started. Where are you? You are NOT at the North Pole.
 43. You have 10 stacks of 10 gold coins. All of the coins in one of these stacks are counterfeit, all the other coins are not. A real coin weighs 10 grammes. A counterfeit coin weighs 11 grammes. You have a modern scale that provides an accurate readout. What is the minimum number of weighings needed to determine which stack is fake?

How about if you had 11 stacks?
cube with a line drawn across two faces to meet at one corner  44. The two red lines in the diagram shown are drawn diagonally on the faces of a cube. What is the angle between them at the point where they join?

(click it to enlarge)
 45. A lady has two children. One is a boy. What are the chances of the other child also being a boy?

How does this change if you are told the oldest child is a boy?
cartoon ant on a triangle  46. Three ants are sitting at the three corners of an equilateral triangle. Each ant randomly picks a direction and starts to move along the edge of the triangle. What is the probability that none of the ants collide?

How about other shapes, Square? N sides?
small watermelon  47. Watermelon is 99% water. I have 100 pounds of watermelon. After a week, drying in the sun, the shrivelled watermelon had only dried down to being 98% water. What is the total weight of the watermelon now?
48. Albert and Bernard just became friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates.

Albert: Then I also know when Cheryl's birthday is.

So when is Cheryl's birthday?

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© Nigel Coldwell 2004 -  – The questions on this site may be reproduced without further permission, I do not claim copyright over them. The answers are mine and may not be reproduced without my expressed prior consent. Please enquire using the link at the top of the page.

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