An old chestnut goes like this:

A 40-mile-long army is marching at a constant speed.

A horseman at the back of the army rides to the front of the army with a message, then immediately turns and rides back to the back of the army.

By the time the horseman reaches the back of the army, the army has marched 40 miles since the horseman started his journey. If the horseman also rides at a constant speed, how far did the horse run during this trip?

(No, the answer is not 40 miles, we want the total distance traveled to the front and back.)


Update: This puzzle appeared as Ken Duisenberg's Puzzle of the Week on February 23, 2003. The original is apparently due to Sam Loyd.

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