Funny Blonde Joke

There was a red haired, a blue haired and blonde stuck on an island. The smart red haired guessed that it was 20 kilometres from where they were and to the city. The red haired swam 5 kilometres before getting tired then swam 5 more but was too tired so she drowned. The blue haired swam 10 kilometres before getting tired then 5 more but was too tired so she drowned. The blonde wondered if they made it so she swam 15 kilometres before getting tired then she said " I'm too tired" and swam back.