A girl was fond of collecting rare stamps. When she was 20 years old, she bought a box to collect her stamps. On her every birthday, she put 250 stamps in the box.

On her sister every birthday (who was also fond of collecting stamps), her sister is allowed to take out 50 stamps from the box. Sadly, when the girl was 60 years old, the girl met with an accident and died.

When they opened her box, there were only 500 stamps in it.

How is this possible logically?