Hey.

As per the guideline to calculate the Top 20 percentile, If the marks in a subject are not allotted out of 100, then the total marks for a subject will be scaled (up or down) to 100 so that the total aggregate marks is out of 500.

So in your case the Marks you scored out of 50 will be scaled upto 100 inorder to calculate the total aggregate out of 500.

Best of luck!