A brainteaser posted by John Derbyshire:
A rope hangs over a pulley. On one end is a weight. Balanced on the other end is a monkey of equal weight. The rope weighs 4oz. per foot. The age of the monkey and the age of its mother together equal 4 years. The weight of the monkey is as many pounds as its mother is years old. The mother is twice as old as the monkey was when the mother was half as old as the monkey will be when the monkey is three times as old as the mother was when the mother was three times as old as the monkey. The weight of the weight plus the weight of the rope is half as much again as the difference between twice the weight of the weight and the weight of the monkey. How long is the rope?