Answer:
20 - 2.55x
Explanation:
20 is the dollar bill Sophie bought the packages with, x represents the number of packages bought and 2.55 is the price for each, so by subtracting 20 by the cost of the total packages results in the amount of change the cashier should give back to Sophie.
For example, if Sophie bought 3 packages of graph paper with a $20 bill the cashier should give back to Sophie $12.35.
20 - 2.55 (3)
20 - 7.65
12.35
hope this helps!