**Linear equations are used to solve problems involving distance, speed and time, especially in transportation and travel.** They are also used to solve problems of weight, mass and density.

A linear equation is one with two expressions that are set equal to each other. It can have one or two variables, and the variables can only be raised to the power of 1. In the case of two variables in an equation, the value of one variable always depends on the value of the other variable.

A common application of linear equations is temperature conversion. In countries that use the Fahrenheit system to measure temperature, such as the United States, linear equations are used to convert temperature from Fahrenheit to Celsius. The equation of a function that converts Fahrenheit to Celsius is an example of a linear equation.

Another common application of linear equations is forex trading. Currency conversion rates are determined using linear equations in which the two currencies being exchanged are the variables in the equation. Telephone call rates are also determined using some form of linear equation. In transportation and travel, linear equations are used to determine the approximate time a journey takes if the speed remains constant.