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.

Here is an arrangement that minimizes changes.  

Each blacksmith changes only once, and works on only two horses.

4 horses are shoed by 1 smith, and the rest by 2.

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