If two quantities are related in such a way that if an increase in one quantity produces a proportional increase in another quantity or if decrease in one quantity produces a proportional decrease in another quantity, then the two quantities are known as direct variations.

When two quantities are related to each other, their changes may follow a certain pattern.

Two quantities x and y are said to be in direct proportion if they increase or decrease together in such a manner that the ratio of their corresponding values remains constant. That is,

where k is a positive number, if x and y are in direct variation or vary directly.