Hiding Behind A Tree
The footpaths on the opposite sides of a street are 30 ft. apart. On one of them a bicycle rider is moving at the rate of 15 mi. per hour. If a man on the other side, walking in the opposite direction, regulates his pace so that a tree 5 ft. from his path continually hides him from the rider, at what rate does he walk?
Source: Herbert Cobb’s Elements of Applied Mathematics, 1911.