In this video the instructor shows how to find a formula describing direct variation between two variables. Variation between two variables can be either direct variation or indirect variation. If a variable y is in direct variation with a variable x we can write the general equation between them as y = k * x. That is as x varies, y varies directly in relation to x, as a multiple of x. Now if you have the values of y and x, you can substitute them in this general equation which gives you the value of the constant k. Now knowing the value of the constant you can find the value of any variable x or y given the value of the other variable. This video shows how to derive the formula describing the direct variation.