A puzzle by Polish mathematician Paul Vaderlind:

If a blacksmith requires five minutes to put on a horseshoe, show how can 8 blacksmiths shoe 10 horses in less than half an hour?

The catch: A horse can stand on three legs, but not on two.

In any 5min interval, 8 of 10 horses can be given a shoe, since we have 8 smiths. Just make sure that no horse is skipped more than once:

Skip horses 1-2 in the first interval and shoe 3-10
Skip horses 3-4 in the next interval
And so on for 5 intervals.

After 5 intervals, each horse will have been skipped exactly once, and so each horse will have gotten a shoe exactly 4 times, and so will be fully shod, taking 25 minutes and meeting the solution conditions.

If you imagine the horses in a circle with the smiths in front of 8 consecutive horses, then after each period, the smiths just move two horses clockwise for the next period.

Posted by Paul on 2015-08-13 11:14:14