Monthly Archives: February 2011

Birthday paradox

In a group of 23 people, the probability that at least two have the same birthday is bigger than 50%. This result may be counterintuitive and probably this is why it is called  ‘paradox’, although it is a real and provable … Continue reading

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Russian roulette

Interview question: “Let’s play  russian roulette. You are tied to your chair and can’t get up. Here’s a gun. Here’s the barrel of the gun, six chambers, all empty. Now watch me as I put two bullets in the gun. … Continue reading

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Boxes and coins problem

Given 10 boxes, each having 10 coins. The weights of the coins from one of the boxes are 10 grams , and the weights of the coins from the other 9 boxes are 9 grams. Using exactly one weighting, how can you … Continue reading

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Compute pi using random numbers

Imagine a square in the plane from coordinates (-1, -1)  to (1,1), and also imagine a circle having radius 1 and origin in (0,0). Generate a random number x in interval (-1, 1), and a random number y also in … Continue reading

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There are more numbers that contain digit 9 than numbers that do not contain digit 9

Remember from the last post that the sum of inverses of all strictly positive integers converges to infinity:  I also proved that the sum of inverses of all strictly positive integers that don’t contain the digit 9 is finite. Denote this … Continue reading

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