Principles of ratio and proportion:

Proportion means to join two equal ratios by the sign of equality (=). For example, a: b = c: d is a proportion between the two ratios. This proportion may also be expressed as a: b: c: d.

In every proportion of two ratios, there are four terms i.e. the first and fourth terms are called extremes, and the second and third are called means. So, in the above proportion 'a' and 'd' are extremes while 'b' and 'care means. The basic rule used to solve problems through ratios and proportion is that the product of the extremes is equal to the product of means. When three values in a proportion are known, the fourth one (X) can be calculated by using this rule.

For example, a biologist can calculate how many birds would get malaria when he allows infected mosquitoes to bite 100 healthy sparrows. In the previous experiment he noted that when he allowed mosquitoes to bite 20 sparrows, 14 out of them got malaria. Now he uses the proportion rule