Five men and five dogs (each man owned a dog) went hiking. They encountered a river that was swift and deep. The only way to cross it was an abandoned boat, left ashore on their side. But it would only hold three living things. Unfortunately, the dogs were edgy and could not be near another person (not even momentarily) unless its owner was present. One of the dogs attended a highly advanced, highly specialized obedience school and therefore knew how to operate the boat (as the men did) -- the other dogs lack this skill. How did the five men and the five dogs cross the river?
Let the five men be represented by A, B, C, D, and E. Let the five dogs be represented by a, b, c, d, and e. Let the dog that can operate the boat be dog a. a, b, and c cross the river. a comes back alone and takes back d. a goes back and stays behind while B, C, and D cross the river. Now A, a, E, and e are on the starting shore, and B, b, C, c, D, and d are on the destination shore. D and d return; A and a cross. C and c go back, and C, D, and E cross. Now dogs c, d, and e are on the starting shore, and everyone else is on the destination shore. a goes back and returns with c and d, then goes back and returns with e.
Today's brain teaser courtesy of Braingle.com.