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.