Algebra: Chapter 12, Lesson 7, page 565.
Joint and Combined Variation
An equation of the form `z = kxy`, where `k` is a constant, expresses joint variation. An equation of the form `z = (kx)/y` expresses combined variation. There are 2 steps involved when solving these equations.
- Find `k`, by using all other information given. If there are 2 other variables, then values for those variables must be provided.
- Using the value of `k` that you calculated, and just one of the other variables with NEW values, you can solve for the unknown variable.
Examples 1, 2 and 3 in the textbook walk you through the steps as well.
This link from purplemath.com has more info too.