OK I think I can do this.
Youâ€™ve got 10 bags. Number then 1 through 10. From bag number 1, take 1 coin, from bag number 2, two coins, bag #3, 3 coins . . . . . and so on up to bag # 10, 10 coins.
Count the coins, 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = 55 coins. Now weigh them all together. If all the coins were gold, they would together weigh 660 oz (12 oz per coin x 55 coins).
However, because some are fake and only weigh 10 oz. the total weight will be less than 660 oz. In fact, you will be short 2 oz for every fake coin you have. So if the total weight is short 2 oz, bag #1 must be fake, because you took one coin from bag # 1. If you are short 4 oz, bag # 2 must be fake since you took 2 coins from it, if you are short 6 oz, bag # 3 must be fake., and so on.
Scott