To solve, set up a proportion
3.25 cups x cups
---------------- = ---------------
13 servings 28 servings
Now cross multiply
13x = 28 * 3.25
Simplify.
13x = 91
Divide both sides by 13
x = 7 cups
To check your answer, use proportions again, but change the left side to 4.5 cups over 18 servings, use the same steps, if the answer is the same as above, then the answer is correct
