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 Description  
 Proportion:  An equation which states that two ratios are equal.

An example of a proportion is:

Proportions can be used to predict information, such as: "a baseball player has a batting average of 0.300 (300 hits in 1000 at-bats). How many hits will she have in 45 at-bats?" We can set up the proportion as:

Note that the numbers representing at-bats are both in the denominator. These numbers could have both been in the numerators; what matters is that you're consistent. That is, numbers representing the same 'thing' are both in the numerator or both in the denominator. One method to solve this proportion is to use the cross product. The cross product is the product of the numerator of one fraction and the denominator of the other fraction. In this problem, the cross products are: 1000h and 300*45.

Set the cross products equal to each other (why?) and solve for the variable. In this problem, the batter will get approximately 13 (h = 13.5) hits.