Need help with math application problem

So here is my question-
The price of a computer fell 20% this year. If the computer now costs $900, how much did it cost last year?

What or how do you do this problem??

LET THE COST OF COMPUTER LAST YEAR BE 100X

THIS YEAR IT COSTS 80X

80X = 900 $

100X = ?

100/80 x 900 = 5/4 x 900 = 4500/4 = 1125 $ answer

1125