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Summary & Examples: Number Series | CSAT Preparation - UPSC PDF Download

Number Series is an important topic in competitive exams, with a specific focus as five questions are dedicated solely to this section. It holds significant importance in various exams. Candidates preparing for competitive exams can secure 4-5 marks in just 2-3 minutes by understanding the different types and patterns of number series in both reasoning and quantitative aptitude.  

What is Number Series?

  • Number series is a form of sequence, where some numbers are mistakenly put into the series of numbers and some number is missing in that series, we need to observe first and then find the accurate number to that series.
  • The questions can also be where you are supposed to determine a continuation to they series or the immediate next term of the series.
  • The sequence is the list of numbers written in a specific order.

Common Types of Number Series

Summary & Examples: Number Series | CSAT Preparation - UPSC

Type 1: Addition / Subtraction or Multiplication / Division

These series follow the pattern of addition or subtraction of even or odd numbers.

Example 1:  What should come in place of question mark in the following series: 19, 23, 39, 75, ?, 239.  

  • The most common trick to solve a number series is to solve by checking the difference between two adjacent numbers, but the difference could not only lead to addition/subtraction but it can also be with multiplication/division.
  • Therefore, to check whether to think addition wise or multiplication wise in increasing sequence, one must assume with the help of the difference between the first and last number of the given sequence.
  • If the difference seems to be less according to the number of steps used to make the last number from first than we should check addition.
  • If the difference seems to be large or too large one must check the multiplication trick between the adjacent numbers.

In Example 1, given above, the difference between first no. (19) and last number (239) is 220.

  • Now a question will arise how we will assume whether the difference is more or less. We will assume it by keeping in mind the number of steps required to start from the first number till the last number.
  • In Example 1, 19 becomes 239 in five steps, because there are four more numbers between them, one of which we have to find out.
  • The difference of 220 between 19 and 239 in five steps logically giving priority to addition over multiplication in an increasing sequence like this.

Summary & Examples: Number Series | CSAT Preparation - UPSC

  • 75 + 64 = 139, 139 + 100 = 239
    139 is correct answer.

Question for Summary & Examples: Number Series
Try yourself:Look at this series: 7, 10, 8, 11, 9, 12, ... What number should come next?
View Solution

Example 2: What should come in place of question mark in the following series: 10, 31, 95, 288, ?, 2609. 

As we can see in the above example, the difference between the first number (10) and the last number (2609) is 2599 in five steps, which indicate us to check multiplication trick between the numbers.
Summary & Examples: Number Series | CSAT Preparation - UPSC

(288*3) + 4 = 868, is the correct answer.

Note : While checking multiplication trick always start from right end of the sequence.

Example 3: What should come in place of question mark in the following series: 30, 34, 43, 59, 84, 120, ?.
(a) 169
(b) 148
(c) 153
(d) 176
(e) None of these

Correct Answer is Option (a)
The given pattern is:

+4, +9, +16, +25 and so on.
So, missing term is 169 = 120 + 49

Example 4: What should come in place of question mark in the following series: 40, 54, 82, ?, 180, 250.  
(a) 142
(b) 124
(c) 136
(d) 163
(e) None of these

Correct Answer is Option (c)
The pattern is: +14, + 28, + 42, + 56, + 70
So, missing term is 82 + 42 = 124


Example 5: Find the wrong number in the below-mentioned series: 
0, 1, 3, 8, 18, 35, 264
(a) 62
(b) 35
(c) 18
(d) 8
(e) None of these

Correct Answer is Option (a)

There is a pattern in difference between the consecutive numbers:

0 - 1  = 1 ← 02 +  1
1 - 3 = 2 ← 12 +  1
3 - 8 = 5 ← 22 +  1
8 - 18 = 10 ← 32 +  1
18 - 35 = 17  ←   42 +  1
Next difference should be  52 + 1   i.e. 26.
35 - 61 = 26 ← 52 +  1
From above the correct sequence would be: 0, 1, 3, 8, 18, 35, 61 and
so, the wrong number is: 264

Example 6: Find the wrong number in the below-mentioned series:
5531, 5506, 5425, 5304, 5135, 4910, 4621
(a) 5531
(b) 5425
(c) 4621
(d) 5135
(e) 5506

Correct Answer is Option (a)

The number should be 5555 in place of 5531.
-72, -92, -112, -132, -152, -172…

Example 7: Find the wrong number in the below-mentioned series: 
6, 7, 9, 13, 26, 37, 69
(a) 7
(b) 26
(c) 69
(d) 37
(e) 9

Correct Answer is Option (b)

The number should be 21 in place of 26.
The pattern is: +1, +2, +4, +8, +16, +32

Example 8: Find the wrong number in the below-mentioned series:
1, 3, 10, 36, 152, 760, 4632
(a) 3
(b) 36
(c) 4632
(d) 760
(e) 152

Correct Answer is Option (d)

The number should be 770 in place of 760.
The pattern is: ×1 +2, ×2 +4, ×3 +6, ×4 + 8, ×5 +10, ×6 + 12, …

Type 2: Perfect Square or Perfect Cube  

Question: 4, 18, 48, 100, 180, ___.

  • In case, if Type-1 is not applicable in a sequence, then in the next step, we must compare given numbers or their differences to square or cube of natural numbers, as in the above example.
    Summary & Examples: Number Series | CSAT Preparation - UPSC
  • Therefore, (7)3-(7)2 = 294, is the correct answer.

(a) Perfect Square Series: 

This type of series is based on the square of a number that is in the same order and one square number is missing in that given series. 

Example 11: In the following options, a few number series are present. One of them has an error, pick the wrong one out: 

(a) 3, 9, 15, 21

(b) 9, 81, 225, 441

(c) 441, 529, 676, 841

(d) 900, 841, 784, 729

Answer:  (c)

Solution: The first series is just an A.P. with a common difference of 6. Now, we can see that the second sequence is the perfect square sequence. This series can be formed from the series given in option A. The third sequence is a two-tier square series but in place of 841, we must have 900. This is a wrong series. The last series is also a square series. So the correct option or the wrong series of the four options presented above is C) 441, 529, 676, 841. 

Example 12: What should come in place of question mark in the following series:
841, ?, 2401, 3481, 4761
Answer: 292, 392, 492, 592, 69

Question for Summary & Examples: Number Series
Try yourself: In the sequence given below, a term is missing. The missing term is written in the options that are present below. Find the missing term and choose it from the options below: 8, 12, 21, 37, __
View Solution

Example 13: What should come in place of question mark in the following series:
1, 9, 25, ?, 81, 121
Answer: 12, 32, 52, 72, 92, 112

Example 14: What should come in place of question mark in the following series:
289, 225, 169, ?, 81
Answer: 172, 152, 132, 112, 92


(b) Perfect Cube Series: 

This type of series is based on the cube of a number that is in the same order and one cube number is missing in that given series.
Example 15: What should come in place of question mark in the following series:
3375, ?, 24389, 46656, 79507
Answer: 153, 223, 293, 363, 433
(Each cube digit added with seven to become next cube number)

Example 16: What should come in place of question mark in the following series:
729, 6859, 24389, ?, 117649, 205379
Answer: 93, 193, 293, 393, 493, 593

Example 17: What should come in place of question mark in the following series:
1000, 8000, 27000, 64000, ?
Answer: 103, 203, 303, 403, 50


Type 3: Factorisation / Prime Factorisation 

  • If Type 1 and Type 2 is not applicable in a sequence, one must try to make factors of the numbers in the next step.

Example 18: 6, 15, 35, 77, 143, __.
In the above example, all the previous tricks are not applicable to get an answer. Hence, We will make factors of the given numbers.
Summary & Examples: Number Series | CSAT Preparation - UPSC(2,3,5,7,11,13) all are prime numbers in ascending order.
Hence, 13*17 = 221, is the correct answer.

Question for Summary & Examples: Number Series
Try yourself:Next number in series 31, 41, 47, 59, __?
View Solution

Type 4: Fibonacci Series 

  • A series in which a number is made by using previous two numbers are called Fibonacci series.

Example 19: 1, 4, 5, 9, 14, 23, ___
In the above sequence, all the numbers are the sum of the previous two numbers.
Therefore, 23+14 = 37, is the correct answer.

Type 5: Sum of Digits 

Example 20:
Summary & Examples: Number Series | CSAT Preparation - UPSCIn the above sequence, the difference between two numbers is the sum of the digits of the first number.
Hence, 89+17 = 106, is the correct answer.

Example 21: 
Summary & Examples: Number Series | CSAT Preparation - UPSCAll the numbers are multiplied by their sum or added by their sum alternately.
Therefore, 11788 + 25 = 11813, is the correct answer.

Type 6: Alternate Pattern Series

  • When numbers given as a hint in a question are more or when a question asks two numbers of a series or the same number come twice in a series, these all give a hint to alternate pattern series.

Example 22: 
Summary & Examples: Number Series | CSAT Preparation - UPSCSo, numbers are 12+5 = 17, 53-6 = 47
Ans. 17, 47

Example 23:
Summary & Examples: Number Series | CSAT Preparation - UPSC
So, the number is 24+3 = 27
Ans. 27 

Type 7: Decimal Pattern Series

When the numbers of the sequence are given in the decimal form is decimal pattern series.
Example 24: 
Summary & Examples: Number Series | CSAT Preparation - UPSCSo, the answer is 18 * 0.8 = 14.4

Example 25:  16, 24, 60, 210, 945, __
The pattern is:
16 * 1.5 = 24
24 * 2.5 =60
60 * 3.5 = 210
210 * 4.5 = 945
945 * 5.5 = 5197.5

Type 8: Bracket Pattern Series

While using bracket pattern we multiply first outside and either add or sub based on given number.

Example 26: 3, 28, 180,  ____,  3676
The pattern is :
(3+1)*7 = 28
(28+2)*6 =180
(180+3)*5 = 915
(915+4)*4 = 3676
Ans. 915

Example 27: 37, 31, 52, 144, __, 2810
The pattern is :
(37-6)*1=31
(31-5)*2=52
(52-4)*3=144
(144-3)*4=564
(564-2)*5=2810
Ans. 564

Type 9: Dual Pattern Series

Example 28:
15,   9,    8,     12,       36,       170
19,   a,    b,     __,       d,           e,
The pattern is :
(15-6)*1=9
(9-5)*2=8
(8-4)*3=12
Similarly:
(19-6)*1=13
(13-5)*2=16
(16-4)*3=36
Ans. 36

Type 10: Factorial Based Series

This is the latest pattern question asked in the latest exams.

Example 29: 
Summary & Examples: Number Series | CSAT Preparation - UPSCAns. 33

Example 30: 
Summary & Examples: Number Series | CSAT Preparation - UPSCAns. 606 + 721 = 1327 

Type 11: Arithmetic Series

A series in which the next term is obtained by adding or subtracting a constant number to its previous term.

Example 1: Find the next term 7, 12, 17, 22, 27, ?
Solution:
The above series is an Arithmetic series where 5 is added to each term to get the next term i.e.

  • 12 = 7 + 5
  • 17 = 12 + 5
  • 22 = 17 + 5
  • 27 = 22 + 5

Summary & Examples: Number Series | CSAT Preparation - UPSCHence next term  will be  27 + 5 = 32.

Example 2: Write the missing term 29, 23, 17, 11, ?
Solution:
The above series is an Arithmetic series where 6 is subtracted from each term to get the next term i.e.

  • 29 – 6 = 23
  • 23 – 6 = 17
  • 17 – 6 = 11

Hence next term will be 11 - 6 = 5.

Type 11: Geometric Series

A series where each successive number is obtained by either multiplying or dividing the previous number by a specific number.

Example 1: Find the missing term 3, 12, 48, 192, ?
Solution:
Here each number is multiplied by 4 to get the next number i.e.

  • 12 = 3 x 4
  • 48 = 12 x 4
  • 192 = 48 x 4

Summary & Examples: Number Series | CSAT Preparation - UPSCHence next term will be 192 x 4 = 768.

Example 2: Find the next term 729, 243, 81, 27, ?
Solution. 
Here each number is divided by 3 to get the next number.

  • 729/3 = 243
  • 243/3 = 81
  • 81/3 = 27

Hence next term will be 27/3 = 9

Type 12: Mixed Series

A series where more than one pattern is arranged in a single series.

Example 1: Find the missing term 2, 8, 26, 80, 242, ?
Solution:
Here the pattern is (x 3 + 2).

  • 2 x 3 + 2 = 8
  • 8 x 3 + 2 = 26
  • 26 x 3 + 2 = 80
  • 80 x 3 + 2 = 242

Summary & Examples: Number Series | CSAT Preparation - UPSCHence missing term will be242 x 3 + 2 = 728

Example 2: Find the next term 5, 12, 27, 59, ?
Solution:
Here each term is multiplied by 2 and consecutive prime numbers are added.

  • 5 x 2 + 2 = 12
  • 12 x 2 + 3 = 27
  • 27 x 2 + 5 = 59

Hence next term will be 59 x 2 + 7 = 125

Type 13: Arithmetic – Geometric Series

A series which is a combination of Arithmetic and Geometric series.

Example: Find the next term 3, 5, 10, 12, 24, 26, ?
Solution:
Here the pattern is (+ 2, x 2, + 2, x 2 …..)

  • 3 + 2 = 5
  • 5 x 2 = 10
  • 10 + 2 = 12
  • 12 x 2 = 24
  • 24 + 2 = 26

Hence the next term will be 26 x 2 = 52

Type 14:  Alternate Series

A series where two series is combined into a single series.

Example 1: Find the next term 2, 3, 5, 8, 8, 13, 11, ?
Solution:
► Here alternate terms starting with 2 forms an Arithmetic series with a common difference as 3. (2, 5, 8, 11)
► Also, alternate terms starting with 3 forms another Arithmetic series with a common difference as 5. ( 3, 8, 13,..)

Summary & Examples: Number Series | CSAT Preparation - UPSCHence next term will be 18.

Example 2: Find the next term 3, 5, 7, 10, 11, 15, 15, ?
Solution:
Here alternate terms starting with 3 forms an Arithmetic series with a common difference as 4. (3, 7, 11, 15)
Also, alternate terms starting with 5 forms a Geometric series with a common ratio as 2. (5, 10, 15)
Hence next term will be 20.

Important Points to Remember

1. If the change is slow or gradual, then it is a difference series.

e.g.

Summary & Examples: Number Series | CSAT Preparation - UPSC

2. If the change is equally sharp, then it is ratio series.

e.g.

Summary & Examples: Number Series | CSAT Preparation - UPSC

3. If the rise is sharp initially, but slows down later, then it is formed by adding squared, or cubed numbers.

e.g.

Summary & Examples: Number Series | CSAT Preparation - UPSC

4. If the series is alternating, then it may be either a mixed series or two different operations going on alternately.

e.g.

Summary & Examples: Number Series | CSAT Preparation - UPSC

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FAQs on Summary & Examples: Number Series - CSAT Preparation - UPSC

1. What are the different types of number series commonly asked in SSC CGL exams?
Ans. The different types of number series commonly asked in SSC CGL exams include Perfect Square or Perfect Cube series, Factorisation/Prime Factorisation series, Fibonacci series, Sum of Digits series, Alternate Pattern series, Decimal Pattern series, Bracket Pattern series, Dual Pattern series, and Factorial Based series.
2. How can one identify and solve a Perfect Square or Perfect Cube number series in SSC CGL exams?
Ans. In a Perfect Square or Perfect Cube series, the numbers in the series are either perfect squares or perfect cubes. To identify and solve this type of series, one can look for patterns in the difference between consecutive numbers to determine if they are perfect squares or perfect cubes.
3. What is the significance of Factorisation/Prime Factorisation number series in SSC CGL exams?
Ans. Factorisation/Prime Factorisation number series involve breaking down numbers into their prime factors. This type of series helps test takers practice their factorisation skills and identify patterns in the prime factors of the numbers in the series.
4. How can one approach solving a Fibonacci series in SSC CGL exams?
Ans. In a Fibonacci series, each number is the sum of the two preceding numbers. To solve a Fibonacci series, one can start with the first few numbers in the series and continue to generate the subsequent numbers by adding the previous two numbers.
5. Why is it important to practice different types of number series for SSC CGL exams?
Ans. Practicing different types of number series helps test takers improve their pattern recognition skills, logical reasoning abilities, and mental math capabilities. It also familiarizes them with the common patterns and sequences that appear in SSC CGL exams.
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