smart blondes?

There were three girls marooned on an island, a red head a brunette & a blonde.

They didn't know what to do except to swim to the other island and get help. The nearest island was 20 miles away.

The red head went first. She swam 15 miles and got tired, and drowned.

So the brunette went next. She swam 17 miles, got too tired and she drowned also.

So the blonde went. She swam 19 miles, got too tired and swam back.