# Number System - Quantitative Aptitude

Some of the key concepts that you need to understand with respect to this chapter is:
1. Number system
2. Even - Odd Numbers
3. Prime Number - Composite number

Most of the topics above, you would already know.
Some additional pointers that you need to know are as follows:

1. There are 25 prime numbers under 100: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97

2. A number is a perfect number if the sum of its factors, excluding itself and but including 1, is equal to the number itself
Eg: 6 (1 + 2 + 3 = 6), 28 (1 + 2 + 4 + 7 +14 = 28)

3. Two numbers are co-prime to each other, if they do not have any common factor except 1.
Eg: Those coprime to 12 are 1 , 5 , 7 , 11 , 13 , 17 , 19 , 23 , 25

4. The number 1 is neither prime nor composite.

5. The number 2 is the only even number which is prime.

Understand more about Divisibility tests and short-cuts/ techniques by views the below video from UrFaculty.com

Here are some more techniques to understand Divisibility tests

Cyclicity of numbers is another important concept that needs to be analyzed: