If there are ten stacks ten coins each and counterfeit coins weigh twice as much as real ones How can you find the stack with counterfeit coins using a scale just once?

Answer 1

if the question allows you to use a scale once, use it in one of 2 ways.

1. Put all stacks of coins on the scale and remove 1 stack at a time, when the weight reaches the the correct amount for the number of stacks are on the scale then then counterfeit has been removed.

2. take 1 coin from stack 1, 2 coins from stack 2, etc. when looking at the numbers if each coin weights, lets say, 1 ounce, and stack 3 is the problem, the scale will read 3 ounces too many.

If you can't use a scale, then sorry, your out of luck.