A 90 oz. bottle of shampoo is sold for $18.95. What is the price per ounce

This sets up as a very commonly used proportion.

90 oz / 18.95 = 1 oz / x Cross multiply

90 x = 18.95 Divide by 90

x = 18.95 / 90

x = 0.21 dollars or 21 cents.

So each ounce of shampoo costs 0.21 dollars or 21 cents.

This is a very handy way to check best deals. Stores have trained us for years to believe that the more we buy of a brand, the better the price. It isn't always true. Sometimes buying the smaller quantity is the better deal. There is only one way to be sure and that's to do a proportion like this one.

Try this is an example. The same company makes a smaller container of shampoo of 45 oz for 9.15. How much is 1 oz and which is the better deal? You should get 0.20333 dollars so this is marginally (just) the better deal.