Q: Start with 100 coins: 10 stacks of 10 coins each.In 9 of the stacks, the coins all weigh 1 ounce but one stack has coins that weigh 2 ounces. Using only a digital scale and only one weighing, identify the stack that contains the 2-ounce coins.

A: Number the stacks from 1 to 10.
Then represent each stack with its number of coins so Stack 1 will have 1 coin and Stack 2 will have 2 coins, etc.
These 10 stacks if all the coins weighed the same would be 55 ounces
(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10).
The difference between the actual weight and 55 would tell us which stack had the 2-ounce coins .
For example. if stack 5 was the stack with the 2-ounce coins, the weight would have been 60 (55+5).