When two variables change in the same sense i.e as one amount increases, the other amount also increases at the same rate it is called direct proportionality. When two variables such as x and y are given, y is directly proportional to x if there is non zero constant k. The constant ratio is called constant of proportionality or proportionality constant.

- If the number of things purchased is increasing then the amount to pay will also increase.
- If the speeds of the car will increase then the time to reach the destination will decreases.

**Variations**

If the value of two objects depends upon each other in such a way that the increase or decrease in the value of one object affects the value of another object then these two objects are said to be in **variation**.

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