Concept of Direct and Inverse Proportion

In mathematics, two quantities are said to be proportional if they vary in such a way that one of the quantities is a constant multiple of the other, or equivalently if they have a constant ratio.
(Wikipedia)

Alright, so what is direct proportion? Direct proportion is when something (must be a number value) is directly related to the proportion something else.

For example, if a car is travelling at at a constant speed of 65km/h, the distance it covers in an hour would be 65km. Therefore, it can be seen that the speed of the car is directly proportionate to the distance the car covers.

Inverse proportion is the exact opposite of direct proportion. So, inverse proportion is when something is inversely related to the proportion of a number value.

For example, let us say that you are driving a car and you are going to travel 70km. Consider this to be a constant distance throughout the following discussion.

1. Suppose that you spent 1 hour driving. Your average speed would be 70 km/h.
   
2. Suppose that you spent 2 hours driving. Your average speed would be 35 km/h.
   So, changing the number of hours that you drive will change the average speed that you will travel.

This is inverse proportion.