In a proportional relationship, the constant of proportionality is the number you can multiply each value of x by to get the related value of y. You can use this idea to figure out whether a set of values is in a proportional relationship. Divide each value of y by the related value of x. Do you get the same number every time?

In our ice cream example, $2 divided by 1 scoop equals 2. $4 divided by 2 scoops also equals 2. And $6 divided by 3 scoops…you get the picture! The relationship’s proportional, and the constant is 2.

Which of these is a proportional relationship?
A - Justin charges $1 for 1 cup of lemonade, $3 for 2 cups of lemonade and $4 for 3 cups of lemonade.
B - Jared charges $1 for 1 cup of lemonade, $2 for 2 cups of lemonade and $3.50 for 3 cups of lemonade.
C - Jasmyne charges $1 for 2 cups of lemonade, $2 for 4 cups of lemonade and $3 for 6 cups of lemonade.
D - Jessica charges $1 for 1 cup of lemonade and $2 for anyone who wants 2, 3 or 4 cups of lemonade.