Alex, Blake, Charlie and Drew 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 up to two swimmers.
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, and must swim at the pace of the slower swimmer.
The shore is 50 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.
Alex can swim the distance in 3 minutes
Blake can swim the distance in 7 minutes
Charlie can swim the distance in 13 minutes
Drew can swim the distance in 17 minutes
How long does the overall rescue take - the answer is not 43 minutes!