Shirt for sale

Question

A shirt is on sale for 10% off. The sale price is $13.50. What was the original price?

Answer 1

SalePrice = OriginalPrice - Discount

or

OriginalPrice = SalePrice + Discount

1. Do you see why the two equations above are equivalent?

2. Can you figure out the Discount and then plug it in to either equation and get the answer?

3. Can you figure out 10% discount without a calculator?