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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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