Two variables are directly proportional if they have a common ratio. This means that the dependent variable is a constant multiple of the independent variable.
The general formula for direct proportion is y = kx. In this formula k is called the constant of proportion or the constant of variation.

An example of direct proportion is the price of gasoline. On a sign advertising the price of gas, the price is given in dollars per gallon, or price per liter. This price is the common ratio. To calculate the total cost, multiply the price per gallon or price per liter times the number of gallons or liters. If the price is $3.00 per gallon, the formula for the function is t(g) = 3g, where g is the number of gallon of gas and t(g) is the total cost of the gas. A synonym for direct proportion is direct variation. 

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