Amy enjoys eating jelly sandwiches and drinking orange juice. She is particular about proportions, though: For every jelly sandwich she eats, she must drink exactly one glass of orange juice, and vice versa. Amy can purchase the jelly for her sandwiches in two jar sizes: 20 ounces and 40 ounces. Amy cares only about the total amount of jelly she has available and not at all about the jar size. In other words, she's just as happy with two 20-ounce jars as she is with one 40-ounce jar. In this scenario, 20-ounce jars of jelly and 40-ounce jars of jelly areperfect substitutes: