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There are many examples where proportions could be used to solve everyday problems. Here are two from my own experience.

 

I am baking a  vanilla birthday cake for my father’s party on Saturday. There will be  33 people at the party who want a piece of cake. The recipe will make a cake with 8 servings. The recipe calls for 3 cups of flour. If there are 4 cups of flour in a pound, how many pounds of flour will I need to buy in order to give all people a piece of cake?     

  1.Set up a proportion:

                                Cups of flour in recipe      =  cups of flour needed

                              servings of cake in recipe        servings needed

 

                                                3 cups                   =         X       

                                                   8 servings                      33

   2. Cross multiply

                                3 X 33 = 8X                             

                                99 = 8X

    3. Divide both sides by 8

                                99÷ 8 = 8X ÷ 8

                                12 .375 = X

 4. I need 12 3/8 or 12.375 cups of flour. Since a pound contains 4 cups, I divide 12 3/8 by 4.           

                                12.375 ÷ 4 = 3.09 pounds

 5. Since I need a bit more than 3 pounds, I will have to by at least 4 pounds of flour.

 

 

 

My family is painting two bedrooms in our home this week and we need to buy paint.  The paint we chose says that 1 gallon will cover 350 square feet. The first bedroom measures 21 feet long by 18 feet wide and the second is 24 feet long and 17 feet wide.

1. Set up a proportion:

 gallons of paint/square feet covered = gallons I need/square feet I need to cover

 To finish my proportion I need to know how many square feet are in the  bedrooms. The formula for area is A = lw or length X width.

                                Bedroom 1    A = 21 X 18               

                                                          A = 378 sq ft

                                Bedroom 2     A = 24 X 17

                                                           A = 408 sq ft

                                Total square feet = 378 + 408 = 786 sq ft

   

2. Cross multiply

                1X 786 = 350X

                786 = 350X

3. Divide each side by 350

                786÷350 = 350X÷ 350

                2.2457142857142857142857142857143 = X                                            

 Round to 2.25 gallons are needed so I must buy 3 gallons of paint.

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