the island

There was a blonde red head and brunette. They are all stuck on an island (100 m away from main land) the brunette says I have had it I am going back so she swims 20 m and says I have had enough so she goes back 2 the island then the red head says I have had enough so she swims only 30m and the goes back then the blonde has a go and gets 50m across but then goes back!!