Say I have a program and it updates the total of a bill for each amount paid by customer. I am having problems with having the total update for each amount subtracted from the previous total amount in a loop (so it never ends and always calculates the new total). What would the code look like for this type of program in a loop?

For example-

Total Bill: 100

Customer enter amount you wish to pay: 20

Total Bill: 80

Customer enter amount you wish to pay: 40

Total Bill: 40