Number each bag one to ten. Take one coin out of the first bag, two out of the second, three out of the third and so on. You will then have 55 coins, which you weigh. This should come to 880 ounces. However, the coins weighing 17 ounces will increase the weight by one ounce for each coin. Therefore, the number of ounces above 880 will indicate the bag containing the heavy coins. For example, if the total weight of the coins is 889 ounces, then the ninth bag will be the one containing the heavy coins.