Sherry purchased a new backpack for her daughter. The backpack was on sale for 20% off the regular price. If Sherry paid $28.80 for the backpack without sales tax, what was the original price?

28.80 +.20X = X

.20X = X - 28.80

1/5 =.20

X/5 = X - 28.80

X = 5X - 144

Set the new X equal to the original X

28.80 +.20X = 5X - 144

172.8 +.20X = 5X

172.8 = 4.8X

36 = X

36$ is the original price

To check,

28.80 + (.20 x 36) = 36

28.80 + 7.2 = 36

36 = 36