Unitary method is the one where we find the value of the required number of quantity by first finding the value of the unit quantity. This can be divided into two types, which are direct variation and inverse variation.

Let us consider the real life situation. If two taps having the same capacity can fill a tank in 30 minutes, how long will it take for a single tap to fill the tank? If there are three such taps how long will it take for the three taps to fill the tank. How do we solve such type of problem? We can easily solve these types of problems if we understand the basic definition and equation of inverse proportion. We shall also discuss with some of the interesting examples on inverse proportion.

An inverse relation is indicated when two quantities are so related that,

(i) an increase in one causes a corresponding decrease in the other and vice versa.

(ii) the ratio of any two values of one quantity is the inverse of the ratio of the corresponding values of the other(or) the product of the two compared quantities remains constant.
Examples:
a. Speed varies inversely as time, As speed increases, time decreases, similarly as speed decreases, time increases.
b. Number of men required to complete the work varies inversely as the time. As the number of men increases, the work can be completed in less time, if the number of men decreases the work will take more time to complete.

Let us study the following table which relates the speed and time to cover the same distance.

Speed (mph)
30
40
60
120
Time (hours)
4
3
2
1

The above table shows the speed taken to travel a given distance and the time taken to cover the same distance. We observe that as the speed increases, the time taken decreases.

Also we observe that the product of the speed and the time remains constant under each column.
(i. e), 30 x 4 = 120
40 x 3 = 120
60 x 2 = 120
120 x 1 = 120

We shall discuss with the inversely proportional formula and the inversely proportional equation in the proceeding sections.