If you would like to know how much would 72 ounces of grapes have cost, you can calculate this using the following steps:
6 pounds of grapes ... $18
72 ounces of grapes = 4.5 pounds of grapes ... $x = ?
72 ounces = 4.5 pounds
6 * x = 18 * 4.5
x = 18 * 4.5 / 6
x = $13.5
The correct result would be $13.5.
