Acake recipe requires 3 1/4 cups of flour for 13 servings and 4 1/2 cups of flour for 18 servings. how much flour is required to make a cake that serves 28?

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

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