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I really need help studying for my math final. I need help solving proportions with variables.

My math final is tomorrow and I can't figure out how to do proportions with variables. I am allowed to use a calculator so that will be ok.

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Rita S. | Hate Reading? Scared of Math? I CAN HELP!!Hate Reading? Scared of Math? I CAN HE...
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Proportions compare 2 things.  For instance, if a questions asks how many cows can graze on 60 acres of a land if 15 cows can graze on 3 acres of land, we set up the proportion as cows : acres  

                          15 : 3 = x : 60

Then convert the equation to a fraction: 

                         cows/acres

                         15     x

                       ___ = ___

                        3        60

Next, cross multiply by multiplying the numerator on one side of the equal sign by the denominator on the other side of the equal sign:

                   15*60 = 3x

                     900 = 3x

Divide both sides of the equal sign by 3 to isolate x

                    900/3 = 3x/3

                       300 = x