Henry Ernest Dudeney/Modern Puzzles/38 - The Despatch-Rider

by : $38$

 * The Despatch-Rider
 * If an army $40$ miles long advances $40$ miles
 * while a despatch-rider gallops from the rear to the front,
 * delivers a despatch to the commanding general,
 * and returns to the rear,
 * how far has he to travel?