The psychologist Max Wertheimer was a friend of Albert Einstein. In 1934, he wrote a letter to Einstein with the following brain teaser.
An old car needs to go up and down a hill. In the first mile–the ascent–the car can only average 15 miles per hour (mph). The car then goes 1 mile down the hill.
How fast must the car go down the hill in order to average 30 mph for the entire 2 mile trip?
