Question

The shampoo you like to use is on sale. But you only get the lower price if you buy 3 bottles...?

Answer 1

The sale price of 3 bottles of shampoo is 3s and the regular price is 3r. The amount of money you save will only come in when you buy 3. The cost you save is the difference between the regular price and the sale price. The equation is therefore,
                         c = 3r - 3s