Three coworkers would like to know their average salary. how can they do it, without disclosing their own salaries?


How about: Person A writes a number that is her salary plus a random amount (AS + AR) and hands it to B, without showing C. B then adds his salary plus a random amount (BS + BR) and passes to C (at each step, they write on a new paper and don’t show the 3rd person). C adds CS + CR and passes to A. Now A subtracts her random number (AR), passes to B. B and C each subtract their random number and pass. After C is done, he shows the result and they divide by 3.


World Series

You have $10,000 dollars to place a double-or-nothing bet on the Yankees in the World Series (max 7 games, series is over once a team wins 4 games). Unfortunately, you can only bet on each individual game, not the series as a whole. how much should you bet on each game, so that, if the … Read More

100 Factorial

how many trailing zeroes are there in 100! (100 factorial)? Solution One per factor of 10, and one per factor of 5 (there are more than enough 2’s to pair with the 5’s), plus one per factor of ten squared (one occurrence) and one per factor of 5 squared (three occurrences).

Oil Mogul

You are an oil mogul considering the purchase of drilling rights to an as yet unexplored tract of land. The well’s expected value to its current owners is uniformly distributed over [$1..$100]. (i.e., a 1% chance it’s worth each value b/w $1..$100, inclusive).


It’s the middle ages, you’re travelling across europe and you want to find the way to vienna. you come to a crossroads, now there are two ways to go. at the crossroads stand a knight and a knave. the knight answers every question truthfully. the knave answers every question falsely. you don’t know which guy … Read More


You find yourself in a duel with two other gunmen. you shoot with 33% accuracy, and the other two shoot with 100% and 50% accuracy, respectively. the rules of the duel are one shot per-person per-round. the shooting order is from worst shooter to best shooter, so you go first, the 50% guy goes second, … Read More


If a hen and a half lay an egg and a half in a day and a half, how many hens does it take to lay six eggs in six days? Solution If 1.5 hens lay 1.5 eggs in 1.5 days (or 36 hours) then: 1 hen lays 1 egg in 1,5 days or 4 … Read More

Box ‘o Numbers

Arrange the numbers 1 to 8 in the grid below such that adjacent numbers are not in adjacent boxes (horizontally, vertically, or diagonally): ___ | 1 | ============= | 6 | 4 | 3 | ============= | 2 | 7 | 5 | ============= | 8 | =====

Coin Problem

some of you may have easily solved the pill weighing problem posed here. If so, you are going to love this problem. It is similar but much more difficult. from my buddy Tom: Ok, here’s a tough one (i thought). There are no “aha!” tricks – it requires straightforward deductive-reasoning.

Parallel Lines

Solution Not really much to solve