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Ten people, a two-seat boat, how many crossings?

river-crossingcountingfencepost

Ten people stand on a riverbank with one rowing boat that holds at most two people. There are no other constraints: anyone can row, and nobody minds who they are with. The boat cannot cross empty, so after each trip over, someone must row it back, until the last trip.

What is the smallest number of one-way crossings needed to get all ten across?

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Each trip over followed by a trip back moves how many people net? And the last trip does not need a return.

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