Introduction
The percentage is a very commonly used term, but there is the fact that most of us don't know! The percentage is a word that is derived from the french word 'cent,' which means 100 in french.
Percentage basically is essentially out of hundred. This means that we compare data and numbers revolving around the value â€ś100â€ť.
For example: If we have a class of 6 students (A1, A2, A3, A4, A5, A6) and these six students have appeared for the 12th board exams, then we can judge which student is good in studies by looking at the percentages of them.
Student  A1  A2  A3  A4  A5  A6 
Percentage  78%  82%  86%  68%  95%  92% 
And now we can conclude that student A5 is the best out of these as the student has the maximum percentage of marks amongst all the students.
Did you know? Whenever you multiply any ratio with 100, it returns the percentage. But how?
Letâ€™s take the help of the unitary method to know this.
Unitary Method: Whenever we have a situation where two variables move in a linear manner with respect to each other, then the unitary method comes into play.
Why not have an example to understand this method?
Example
Raju goes to the market and buys ten apples for rupees 30. How much will he pay for 15 apples?
Solution
let Y be the rupees you will pay for 15 apples, Then, 10 apples = 30 Rs 15 apples = y Rs Cross multiplying to equate; 10 X Y = 15 X 30 Y = 45
So, we will pay 45 rupees for 15 apples. 
Letâ€™s see how we can implement the unitary method in the percentagebased questions:
Example
8% of a number is 6000. What is the number?
Solution
Let the number be N, Now, If 8% of N is 6000 Then 1% of N will be 6000/8 that is 750. And this results in 100% of N being 750 X 100 = 75000 Hence, Using the unitary method we can say that N = 75000 
Now, as we have understood percentage so let's see the concept of the percentage change.
Percentage Change
The term percentage always occurs when we go from one number to another. But the basic structure of percentage change will only and only occur when we talk about the difference between two numbers
Letâ€™s understand this with an example:
If we have a number x that is changing to y
then the percentage change of x to y will be
Percentage change = (change / original value) * 100

Let's see this actual using numbers.
If we have two numbers, 40 and 60, and we are going from 40 to 60, then we will have the percentage change as follows:
Change = 6040 = 20 Original value = 40 Then, Percentage Change = (20 / 40) * 100 = 50% The percentage is increasing by 50% 
But if we have the same two numbers and the change this time is from 60 to 40 then the change is as follows:
Change = 4060 = 20 Original value = 60 Then, Percentage Change = ( 20 / 60) * 100 = 16.66% Percentage is decreasing by 16.66% 
NOTE: 1. In percentage change, there should always be two numbers.
2. we need to figure out which number is the original number.