What types of situations might be modeled with equations containing two or more variables?

Equations containing two or more variables are used to model situations where two or more unknown values need to be determined. For example, if a car manufacturer manufacture a new car model and want to track its costs and revenues, he can use equations with two variables to do this. A system of linear equations which consist of two or more linear equations [depending on the variables to determined] made up of two or more variables, so that all equations in the system are considered simultaneously can be used for this kind of situation.