Gold

A gold bar balances with nine-tenths of 1 pound and nine-tenths of a similar gold bar. How much does each gold bar weigh?

Answer:

Nine pounds. Examine the objects on the right side of the balance. If we looked at the balance pan containing the two bars, we'd see that one-tenth of the gold bar is absent. In its place we have nine-tenths of a pound. From this we can infer that one-tenth of a gold bar weighs nine-tenths of a pound. Therefore, a complete gold bar would weigh ten times as much. 9/10 pound x 10 = 90/10, or 9 pounds.