Mrs. davis bought 6 pounds of grades for $18. at this rate, how much would 72 ounces of grapes have cost?

This the problem we are looking at: 6 pounds/18 = 72 ounces /?
To solve this, we have to make the unit of measurement the same. There are 16 ounces in 1 pound. So, there are 72 ounces in 4.5. To find that you just do 72 divided by 16. This is the new problem we are looking at: 6 pounds/18 = 4.5 pounds/?
To find ?, you must divided 6 divided by 4.5 to find the difference between the two to keep the same rate. That equals 1.33333333. 
What is done to the numerator, you must do to the denominator to keep the same rate so now you divide 18 divided by 1.33333333 which equals 13.5

So for 72 ounces (4.5 pounds) it will cost $13.50

I hope this helps:)