In a snack bar there were two baskets of pears of different sorts, 60 pears in each. It was proposed to sell pears from the first basket at 4 for 30¢, and pears from the second basket at 6 for 50¢. Thus the two baskets of pears would be sold for \$9.50 in all.
However, the cashier, in order to simplify her work, mixed together the pears of both kinds and proceeded to sell ten pears for 80¢.
As a result of the sale of pears, there turned out 10¢ too much: not \$9.50 but \$9.60.
Where did this extra dime come from?
(Source: Lapses in Mathematical Reasoning by V. M. Bradis, V. L. Minkovskii, A. K. Kharcheva.)