Step 1: Take the total amount of payment (100% of it) and divide it by the percent that isn't discounted,like so for example:

Sales total: $20.00

discount: 45%

percent of payment after discount: (100-45) 55%

total sale price times percent after discount (20x.55)

^yes thats a decimal

total amount due: $11.00

or if the total is 20 dollars,there is a $5 dollar discount for every 5%,or even simpler, 1% (in this case) is 1 dollar off.

Sales total: $20.00

discount: 45%

percent of payment after discount: (100-45) 55%

total sale price times percent after discount (20x.55)

^yes thats a decimal

total amount due: $11.00

or if the total is 20 dollars,there is a $5 dollar discount for every 5%,or even simpler, 1% (in this case) is 1 dollar off.