A proportion is a statement of equality between two or more ratios. Thus,

This is read 3 is to 4 as 6 is to 8. The first and last terms of the proportion are called the extremes. The second and third terms are called the means.

In any proportion, the product of the extremes is equal to the product of the means. In the proportion

2:3 = 4:6

the product of the extremes, 2 x 6, is 12; the product of the means, 3 x 4, also is 12. An inspection of any proportion will show this to be true. This rule simplifies the solution of many practical problems.

An airplane flying a distance of 300 miles used 24 gallons of gasoline. How many gallons will it need to travel 750 miles?

Sixty gallons of gasoline will be required to travel a distance of 750 miles.