# Study Material for SSC CGL, CHSL, MTS, Prasar Bharti, FCI Exams : Number System

## Number System

## Place Value (Indian)

**Face Value and Place Value of a Digit**

**Face Value: **It is the value of the digit itself eg, in 3452, face
value of 4 is ‘four’, face value of 2 is ‘two’. **Place Value: **It is the face value of the digit multiplied by the place
value at which it is situated eg, in 2586, place value of 5 is 5 × 102 = 500.

**Number Categories**

**Natural Numbers (N):** If N is the set of natural numbers, then we write N =
{1, 2, 3, 4, 5, 6,…}

The smallest natural number is 1.

**Whole Numbers (W):** If W is the set of whole numbers, then we write W = {0, 1, 2,
3, 4, 5,…}

The smallest whole number is 0.

**Integers (I): **If I is the set of integers, then we write I = {– 3, –2, –1, 0,
1, 2, 3, …}

**Rational Numbers:** Any number which can be expressed in the form of p/q, where p
and q are both integers and q # 0 are called rational numbers.

e.g. 3/2,7/9,5,2

There exists infinite number of rational numbers between any two rational
numbers. Irrational Numbers Non-recurring and non-terminating decimals are
called **irrational numbers.** These numbers cannot be expressed in the form of p/q
.

e.g. √3, √5,√29

**Real Numbers: **Real number includes both rational
and irrational numbers.

**Basic Rules on Natural Numbers**

1. One digit numbers are from 1 to 9. There are 9 one digit numbers. ie, 9 ×
10^{0}.

2. Two digit numbers are from 10 to 99. There, are 90 two digit numbers. ie, 9 ×
10.

3. Three digit numbers are from 100 to 199. There are 900 three digit numbers ie,
9 × 10^{2}.

In general the number of n digit numbers are 9 × 10^{(n–1)}

Sum of the first n, natural numbers ie, 1 + 2 + 3 + 4 + … + n = n n 1 / 2

Sum of the squares of the first n natural numbers ie. 1^{2} + 2^{3}
+ 3^{2} + 4^{2} + …+ n^{2} = n n 1 2n 1 / 6

**Different Types of Numbers**

**Even Numbers: **Numbers which are exactly divisible by 2 are called even
numbers.

eg, – 4, – 2, 0, 2, 4…

Sum of first n even numbers = n (n + 1)

**Odd Numbers:** Numbers which are not exactly divisible by 2 are called odd
numbers.

eg, – 5, –3, –1, 0, 1, 3, 5…

Sum of first n odd numbers = n^{2}

**Prime Numbers: **Numbers which are divisible by one and itself only are called
prime numbers.

eg, 2, 3, 5, 7, 11…

- 2 is the only even prime number.
- 1 is not a prime number because it has two equal factors.
- Every prime number greater than 3 can be written in the form of (6K + 1) or (6K – 1) where K is an integer.
- There are 15 prime numbers between 1 and 50 and l0 prime numbers between 50 and 100.

**Relative Prime Numbers: **Two numbers are said to be relatively prime if
they do not have any common factor other than 1.

eg, (3, 5), (4, 7), (11, 15), (15, 4)…

Twin Primes: Two prime numbers which differ by 2 are called twin primes.

eg, (3, 5), (5, 7), (11, 13),…

Composite Numbers Numbers which are not prime arc called composite numbers

eg, 4, 6, 9, 15,…

1 is neither prime nor composite.**Perfect Number: **A number is said to be a perfect number, if the sum of
all its factors excluding itself is

equal to the number itself. eg, Factors of 6 are 1, 2, 3 and 6.

Sum of factors excluding 6 = 1 + 2 + 3 = 6.

6 is a perfect number.

Other examples of perfect numbers are 28, 496, 8128 etc.

**Rules for Divisibility**

**Divisibility by 2: **

eg, 3582

**Divisibility by 3:**

eg, 453 = 4 + 5 + 3 = 12.

12 is divisible by 3 so, 453 is also divisible by 3.

**Divisibility by 4: **A number is divisible by 4, if the number formed
with its last two digits is divisible by 4. eg, if we take the number 45024, the
last two digits form 24. Since, the number 24 is divisible by 4, the number
45024 is also divisible by 4.

**Divisibility by 5:**

eg, 10, 25, 60

**Divisibility by 6:**

eg, 48, 24, 108

**Divisibility by 7: **

eg, 658

**Divisibility by 8:**

**Divisibility by 9:**

eg, 684 = 6 + 8 + 4 = 18.

18 is divisible by 9 so, 684 is also divisible by 9.

**Divisibility by 10: **. eg, 20, 180, 350,….

**Divisibility by 11:**

eg, 30426

## Division on Numbers

In a sum of division, we have four quantities.

They are (i) Dividend, (ii) Divisor, (iii) Quotient and (iv) Remainder. These
quantities are connected by a relation.

(a) Dividend = Divisor × Quotient + Remainder.

(b) Divisor = (Dividend – Remainder) ÷ Quotient.

(c) Quotient = (Dividend – Remainder) – Divisor.

**Factors and Multiples**

Factor: A number which divides a given number exactly is called a factor of
the given number,

eg, 24 = 1 × 24, 2 × 12, 3 × 8, 4 × 6

Thus, 1, 2, 3, 4, 6, 8, 12 and 24 are factors of 24.

• 1 is a factor of every number

• A number is a factor of itself

• The smallest factor of a given number is 1 and the greatest factor is the
number itself.

• If a number is divided by any of its factors, the remainder is always zero.

• Every factor of a number is either less than or at the most equal to the given
number.

• Number of factors of a number are finite.

**Number of Factors of a Number: **If N is a composite number such that N
= a^{m} b^{n} c^{o}... where a, b, c ... are prime
factors of N and m, n, o ... are positive integers, then the number of factors
of N is given by the expression (m + 1) (n + 1) (o + 1)