Wednesday, March 21, 2007

"I want to get to that island" problem

This is one of the fun logic problems that I encountered when I was still in high school. If you are from Hong Kong most likely you have heard of it already. Here you go:

There is a family on one island. For some reason, they want to get to another island which can be reached by a boat. The family is consisted of
  • Dad
  • Mom
  • Two sons
  • Two daughters
  • A servant
  • A dog
They only have one boat, which can accommodate two people (the dog is treated as a person here). Among the family members, only Dad, Mom and the servant know how to ride the boat. No one knows how to swim (including the dog). The family members must leave the boat once they reach an island, even though they may immediately go on the boat again. Now, there could be some killing actions among the family members:
  • If Dad is not present, Mom will beat the two sons to death.
  • If Mom is not present, Dad will beat the two daughters to death.
  • If the servant is not present, the dog will bite all the people to death.
The killing can happen on the boat or any of the two islands. Now, how should we move all the family members to another island such that no person (including the dog) dies?

If you know the answer already, keep quiet! If you don't, have fun :)