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 find which box contains the coins with 10 grams weight?
Solution (drag over the blank space below to see it):
Take 1 coin from first box, 2 coins from second box, … , 10 coins from the box number 10, and let S be the result of weighting.
Then S – (9 + 18 + … + 81 + 90 ) will be the number of the box that contains coins weighting 10 grams.