Both children cross and one brings the boat back. The man crosses alone and the other child brings it back. That is four crossings to move one adult, and four more do the same for the woman. Then the two children cross together: nine.
A man, a woman and their two children have to cross a river. The man and the woman weigh the same, the boat can carry the weight of one adult, and each child weighs half as much as an adult. What is the fewest crossings it takes to get all four over?
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Two children together weigh the same as one adult.
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Nine crossings
Source: Attributed to Alcuin of York, Propositiones ad Acuendos Juvenes, Proposition 19, c. 800