This puzzle class has several subclasses:
A well known example:
A farmer has a boat, a cabbage, a goat and a wolf. He has to cross the river with his boat. His boat is small, so he can take either the cabbage, the goat or the wolf with him  but not two of them at the same time. If he leaves the goat with the cabbage unattended, the goat will eat it. Likewise, he cannot leave the goat with the wolf alone.
How many crossings must the farmer make?
Warning: Wikipedia contains spoilers!
Clearly the farmer first can take the goat only. After subsequently bringing one of the other two, say the cabbage, the clue is to take the goat temporarily back. He then brings the wolf and finally the goat again. Thus there are seven crossings, four forward and three back.
The famous British puzzler Henry Dudeney[?] added several puzzles to this category.
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