There are ten piles of coins, and ten coins in each pile. Each coin in each pile weighs 1 gram, except for one pile where the coins weigh 1.1 grams.
You have a digital scale, and you are allowed one attempt at using it. You can take any number of coins from any number of piles. How do you determine which pile has the heavier coins?
Answer
You take one coin from the first pile, two coins from the second pile, and so on until the ninth pile, from which you take nine coins. When you put these on the scale, look at the first number after the decimal point. If it shows a one, then you know that the coins in the first pile weighs more, and so on. If it shows a zero, then pile ten is the answer.