Explanation:
The digital scale has to be used once.
Let the coins that have normal weights are normal coins and the coins that has a weight 1 pound less are called defected coins.
5 stacks are there in which any four contains normal coins and 1 stack contains defected coins.
Took 1 coin from 1st stack, 2 coins from 2nd stack and so on..
Weigh the coins and see the result.
If the total weight is 1 pound less than the stack 1 contains defected coin.
If the total weight is 2 pounds less than the stack 2 contains defected coin.
If the total weight is 3 pounds less than the stack 3 contains defected coin.
If the total weight is 4 pounds less than the stack 4 contains defected coin.
If the total weight is 5 pounds less than the stack 5 contains defected coin.