GMAT Quantitative Review
3.0 Math Review
5. Ratio and Proportion
The ratio of the number a to the number b .
A ratio may be expressed or represented in several ways. For example, the ratio of 2 to 3 can be written as 2 to 3, 2:3, or . The order of the terms of a ratio is important. For example, the ratio of the number of months with exactly 30 days to the number with exactly 31 days is , not .
A proportion is a statement that two ratios are equal; for example, is a proportion. One way to solve a proportion involving an unknown is to cross multiply, obtaining a new equality. For example, to solve for n in the proportion , cross multiply, obtaining ; then divide both sides by 3, to get .