For the sake of clarity, I am assuming that each real coins weigh 10g and the fake coins weigh 9g (it does not really matter what they weigh though) Take 1 coin from stack 1; 2 from stack 2; 3 from stack 3; 4 from stack 4; 5 from stack 5; 6 from stack 6; 7 from stack 7; 8 from stack 8; and, 9 from stack 9. This makes a total of 45 coins. This combination of coins give different weight depending upon which stack the coin is in. For example, if the fake stack is stack 1, the total weight will be 449g. ---- Try the same problem, now, using a BEAM BALANCE. Try to find the fake stack by weighing no more than 2 times.