Proportions Fast Facts

When one ratio is set equal to another ratio, it is said that they are proportional.

1/2 and 6/12 can be said to be proportional.

You can test for proportionality (equality) several ways.

You can compare the cross products.

Using cross products, we'd ask ourselves if four times five equaled three times seven. Since it does not, we can assert that the two fractions are NOT proportional.

You can reduce each fraction.

Since both 9/12 and 15/20 reduce to 3/4, we can assert that the two fractions ARE proportional.

Sometimes one of the parts of the proportion is missing. The Fundamental Property of Proportions states that the product of the MEANS is equal to the product of the EXTREMES.

So to look at a more concrete example, we see that we can set the cross-products equal, then solve the resulting equation.

Proportions are often helpful when solving word problems. If you hear two comparisons of like quantities within a word problem, try setting up a proportion.

Ex. If two cups of flour are needed to bake one loaf of bread, then how many cups of flour are needed to bake 5 loaves of bread?

That's an easy one, but notice the comparison: cups of flour to loaves of bread.

So 10 cups of flour would be needed to bake 5 loaves of bread.

I think I'm ready for a Proportions Quiz!

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