Percy used to have 100% of the shirts (before he started selling them). Since then, he has given away 90% of the shirts and is now left with only 10% of what he used to have. He currently has 25 shirts, meaning that 25 is 10% of what he had.
Lets set up a proportion:
25 shirts : 10%
x shirts : 100%
x = 25*100 / 10
x = 2500 / 10
x = 250 shirts
Percy started off with 250 shirts.
