Tricky Math Puzzle to Make you Think Outside the Box

This is Tricky Mathematical Puzzle which will make you think outside the box. It is a tricky math puzzle because just doing Mathematical calculations will not solve this #puzzle. One needs to think outside the box to get started with the correct maths equations which can be later solved to get the correct answer. Let us see if you can solve the tricky math puzzle?

$One girl Sita buys a box when she is 20 years old. On the day of every birthday of her, she will put $250 in this box. She is having one little sister Pooja. On Pooja's birthday, she will remove $50 from the same box to give a gift to her sister. Sita dies at the age of 60. When the box is opened after her death it contains only $500. How?$