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 -- the other dogs lack this skill.
How did the five men and the five dogs cross the river?
Call the man A, B, C, D, and E, and their dogs a, b, c, d, and e. Assume that dog a is the one that knows how to row the boat.
The list of crossings (including each intermediate state):is furnished hereunder as follows:
ABCDEabcde
ABCDEde abc →
ABCDEde abc
ABCDEde ← a bc
ABCDEade bc
ABCDEe ad → bc
ABCDEe abcd
ABCDEe ← a bcd
ABCDEae bcd
AEae BCD → bcd
AEae BCDbcd
AEae ← Bb CDcd
ABEabe CDcd
BEbe Aa → CDcd
BEbe ACDacd
BEbe ← Cc ADad
BCEbce ADad
bce BCE → ADad
bce ABCDEad
bce ← a ABCDEd
abce ABCDEd
e abc → ABCDEd
e ABCDEabcd
e ← a ABCDEbcd
ae ABCDEbcd
ae → ABCDEbcd
ABCDEabcde
Edited on June 19, 2023, 11:09 am
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