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'Whitewater Rafting' printed from http://nrich.maths.org/
Two adults and two children wish to cross a river. They make a raft but it will carry only the weight of one adult or two children. What is the minimum number of times the raft must cross the river to get all four people to the other side? (N.B. The raft may not cross the river without at least one person on board.)
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This problem is taken from the UKMT Mathematical Challenges.