This problem outlines how Eratosthenes of Cyrene approximated the diameter of the earth in approximately 200BC. By observing the shadow of the sun at noon he recognized that his home town of Alexandria was approximately $7.2^\circ$ north of the town of Syene. He paid somebody to walk and measure the distance between Syene and Alexandria. Suppose it is 787 km. Based on these figures, the circumference of the earth is kilometers.
Hint: Observe that $7.2^\circ$ is to 360 degrees as walking 787 km is to walking the entire circumference.