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