Ratios

Two quantities are in direct proportion when they increase and decrease at the same rate. If one doubles, the other doubles too. The relationship is written y = kx, where k is the constant of proportionality.

Two quantities are in inverse proportion when one increases as the other decreases by the same factor. The product of the two quantities is always constant, and the formula is y = k/x.

To divide a quantity in a given ratio, find the value of one part by dividing the total by the sum of the ratio parts, then multiply each ratio number by one part.

A ratio compares two quantities. A proportion states that two ratios are equal. Proportions are solved using cross-multiplication.