Mother Thrifty and her daughter Dollie each drive 30 ducks to the market. Mother Thrifty is eager for a trade, and hurries on with her 30, and reaches the market early when ducks are in demand. She sells two for one dollar; but Dollie dawdles along the road, and when she arrives the market is supplied so she only receives one dollar for every three.
On the way home the mother says, "I got a dollar for two, and you a dollar for three, which makes two dollars for five ducks; but sixty is twelve times five, so we must have twelve times two dollars or twenty-four," but on counting their money they found they had twenty-five dollars.
Why this discrepancy?