Adrian, Bernie, Ceiren and Darren are all trapped on an island in the middle of a crocodile infested lake. Luckily they have one crocodile repelling stick that just about protects any two swimmers from certain death. Thus to get to safety a maximum of two swimmers can be in the water at any one time and they have to be together to benefit from the stick. The shore is 100 metres away and they can not throw the stick back, someone has to swim back with the stick until all four are safely on the shore. If Adrian can swim the distance in 3 minutes, Bernie can swim the distance in 7 minutes, Ceiren can swim the distance in 13 minutes and Darren can swim the distance in 17 minutes, how long does the overall rescue take? The answer is not 43 minutes!

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Adrian and Bernie swim to shore 7 minutes
Adrian returns 3 minutes
Ceiren and Darren swim to shore 17 minutes
Bernie returns 7 minutes
Adrian and Bernie swim to shore 7 minutes
Total 41 minutes.