231. Henry's Walk. Henry went for a walk in the countryside sometime between 8 and 9 in the morning, when the hands of his clock were aligned. He arrived at his destination between 2 and 3 in the afternoon; at this time, the hands of his clock were pointing in exactly opposite directions. How long did Henry's walk last?
231. The walk lasted 6 hours. Suppose we extended the hour hand in the opposite direction. If at the beginning of the walk the hour and minute hands were coincident, then after six hours the hour hand and its extension will simply have swapped places, while the minute hand, having completed exactly six full circles, will return to its initial position and, consequently, will coincide with the extension of the hour hand.

[Ch. Salkind, M. M., 28, 241 (March 1955).]