# Constant of Variation

The constant of variation in a direct variation is the constant (unchanged) ratio of two variable quantities.

The formula for direct variation is

where k is the constant of variation.

Example 1:

If y varies directly as x and y = 15 when x = 24, find x when y = 25.

Find the constant of variation.

To find x, substitute 25 for y.

The constant of variation in an indirect variation is the constant (unchanged) product between two variable quantities.

The formula for indirect variation is

where k is the constant of variation.

Example 2:

If it takes 4 hours at an average speed of 90 km/h to do a certain journey, how long would it take at 120 km/h?

Find the constant of variation.

k = speed · time

k = 90 · 4 = 360