20.8478066 (or 20.85%) you do 480.77 minus 380.54 and it comes out to 100.23 and then you do 100.23/480=x/100 (discount/original=percent/100) and you multiply 100.23 by 100 and it comes to 10023 and you do 10023 divided by 480.77 and it comes out to 20.8478066 or 20.85%
