There are three sisters stranded on a island. They live there until they run out of food. They decide to try to swim to the next island but it is 100 miles away.

The oldest sister swims the first 15 miles, gets tired, and swims back. The middle one swims the first 25 miles, gets tired, and swims back.

The youngest sister, and not the smartest one, swims the first 50 miles, gets tired, and swims back.