# Quantitative Developer Interview Questions

Quantitative developer interview questions shared by candidates

## Top Interview Questions

How to measure 9 minutes using only a 4 minute and 7 minute hourglass http://www.abc.net.au/science/surfingscientist/pdf/conundrum9.pdf http://www.ronbarnette.com/Zeno/result7.html The key is understanding that you will have to use the two hourglasses together. Since this problem could be asked in many ways using different values for the hourglasses and the total amount of time, it's more important to understand how you use the tools rather than memorize a specific example. The question is used to determine those who can apply their knowledge to solve problems vs. those who memorize answers "from the book". Start both timers. After four minutes, the four-minute timer will have expired and the seven-minute timer will have three minutes remaining. Flip the four minute timer over. After seven minutes, the seven-minute timer will have expired and the four-minute timer will still have one minute left. Flip the seven-minute timer over. After eight minutes, the four-minute timer will have expired for the second time. The seven-minute timer will have accumulated one minute after it's last flip. Flip over the seven-minute timer and when it expires nine minutes will have elapsed. For extra measure, you can always throw in something like, "assuming the timers can be flipped over nearly instantly..." Show More Responses |

How many numbers between 1 and 1000 contain a 3? |

You have a chest of 8 drawers. With probability 1/2, you put a letter in one of the drawers. With probability 1/2, you don't put a letter in any drawer. I open the first 7 drawers, all are empty. What is the probability there is a letter in the 8th drawer? |

If you toss a coin n times, what's the expected value of n if you get the 2nd head? |

Given a m*n matrix with values -1 or 1, try to flip the values in a given row and a given line efficiently. |

How to compute square root of integer with only add, sub, div and mult |

Implementing the LRU algo |

Are you familiar with the Singleton or Factory pattern? |

You and I will play a game. I lay out N coins of different denominations (i.e. nickels, dimes, quarters, pennies) in a straight line across the table. We each take turns choosing one coin from either side of the line - meaning, you can only choose one of the end coins. We keep picking coins until none are left. The winner is the person whose sum of coins is the greatest. I let you decide if you go first or second. Do you wish to go first or second? And what is your strategy? |

The most unexpected questions was how would you color a 3-colorable graph. I was initially thinking that you had to do something efficient, which isn't possible, but he just wanted some way of doing it. |

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