We have 5 boxes with 20 coins each. And coins of three boxes weigh 10, one of them weighs 9 and the other weighs 11. How can we differentiate between them by using a scale(digital, not the one with two cups) only once, with 1 unit accuracy?
Place 1 coin from the first bag on the scale 2 coins from the second bag, 4 coins from the third bag, 8 coins from the fourth bag and 16 coins from the fifth bag
subtract 310 from the total, and the result will tell you which boxes contain the wrong coins. (it is I - J, where I and J are 2^n and n is the bag number)
Assuming each coin weigh 10 for 3 boxes, 9 for another box, 11 for the other one.
This is pretty straight forward question. Just choose $0,1,3,7,12$ and this is also the minimum number of coins you can choose as well. You will get different $20$ results except the boxes with coins with the same weight coin.