To find your change, first you must determine how much the customer is paying. To find this, you must multiply the cost of one book by the number of books bought.
$0.75 • 9 = $6.75
To find the change, you must subtract $6.75 from $20. $13.25 is your change.
An example of an equation used to solve this would be
0.75(9) + c = $20