An 8-ounce cup of gourmet coffee costs $3.60. If the unit price remains the same, how much would a 20-ounce cup of coffee cost?

We can solve this problem by several ways, what we're going to do is calculating how much does a ounce cup cost, and, if we know that an 8-ounce cup costs 3.60 dollars, to know how much does one cost, we divide

3.60/8 = 0.450

Now, we know each ounce costs 4.50 and we can multiply the price by 20 to know how much will it cost:

0.450 * 20 = 9 dollars

You could also have done this problem by doing this:

8 ---- 3.60

20 --- x

And solve: x = 9