Driving along the highway, Mr. Smith notices that signs for Flatz beer appear to be spaced at regular intervals along the roadway.
He counts the number of signs he passes in one minute and finds that this number multiplied by 10 gives the car’s speed in miles per hour.

Assuming that the signs are equally spaced, that the car’s speed is constant, and that the timed minute began and ended with the car midway between two signs, what is the distance from one sign to the next?.

Martin Gardner, 1956