"Averages, Percentages and Ratio and Proportion" is one of the easiest topics in quantitative section in CAT. The theory involved in this section is just an extension of high school mathematics. It is for this reason that questions from these topics are often asked in conjunction with other topics. The fundamentals of this topic are hence important not just from a stand-alone perspective, but also to answer questions from other topics.
If a, b, c, d and x are positive integers such that $$\frac{a}{b}=\frac{c}{d}$$
If x is directly proportional to y => x = ky
If x is directly proportional to square of y => x = $$ky^2$$
If x is directly proportional to cube of y => x = $$ky^3$$
Here, 'k' is a proportionality constant.
When x is indirectly proportional to y => $$x=\dfrac{k}{y}$$