The water has been rising since midnight. Ten people are on the rooftop, and there's a rowing boat with two seats. There's time for three trips before the current takes the boat.
Choose who goes on each trip. There's no undoing it, and no combination gets everyone out.
There's no good solution to this. Ten people are on the roof, the boat has two seats, and there's time for three trips. However well you do it, two are going to be left up there. All you get to decide is by what criterion — and that criterion is going to be written down.
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