How to Solve Questions Based on Number Series – Reasoning

We conducted a poll asking for "What would you like to read?". The results were in favour of Mental Ability and Quantitative Aptitude. Thank you for your votes, and here we are to prove that your opinion counts. This is our first post on Mental Ability covered under the Topic Reasoning.

How to Solve Questions Based on Number Series

(FOR BANK PO, SSC, B. Ed. ASPIRANTS)

Most of the number series can be solved by addition and subtraction (Multiplication and Division being the Supersets for these). In plain English that means you need to find what to add or subtract to find the next (or missing) number in the series.

We will understand the concept by solving a few questions and then you can attempt some on your own.

Example 1: Find the next number in the series: 1, 3, 6, 10, 15, __

Answer:

Looking at the series we find that the series is an "INCREASING" sequence.

So we try to find what is being added. Here the series is

1, 1+2, 1+2+3, 1+2+3+4

or Add 1, 2, 3, 4 and so on to the last number.

1+2 =3, 3+3 = 6, 6+4 = 10, 10+5 = 15

So the answer is 15+ 6 = 21.

Practice Question 1: 21, 25, 33, 49, 81, __

(a) 145 (b) 129 (c) 113 (d) 97

Ans (a)

Example 2: Find the next number in the series: 87, 80, 74, 69, 65, __

Answer:

Looking at the series we find that the series is a "DECREASING" sequence.

So we try to find what is being subtracted. The series is

87, 87 - 7 = 80, 80 - 6 = 74, 74 - 5 = 69, 69 - 4 = 65

So the answer is 65 - 3 = 62

Practice Question 2: 66, 36, 21, __

(a) 13.5 (b) 14.5 (c) 13 (d) 14

Ans (a)

Example 3: Find the next number in the series: 40, 15, 30, 25, 20, 35, __

Answer:

Looking at the series we find that the series is a "MIXED" sequence.

By mixed series we mean that it is composed of two different series.

i.e. 40, 30, 20, ... (decreasing) and 15, 25, 35, .. (increasing)

The trick here is to know which series will give us the answer. We see that the missing number belongs to the decreasing series. Ignoring the other series the answer is

20 - 10 = 10

Practice Question 3: 7.5, 5, 15, 4, 30, 3, 60, 2, __

(a) 1 (b) 75 (c) 120 (d) 135

Ans (c)

Some Sample Questions (Easy to Difficult, No Options Given for Now, you don't need them)

1. 8, 20, 32, 44, __

2. 55, 44, 33, 22, __

3. 63, 57, 52, 48, __

4. 100, 85, 75, 70, __

5. 96, 102, 114, 132, __

6. 17, 26, 44, 80, __

7. 10, 13, 12, 15, 14, __, __ (two missing numbers)

8. 12, 7, 18, 10, 24, 13, __

9. 2, 5, 11, 23, 47, 95, __

10. 1, 4, 9, 16, 25, 36, __

We will continue with this post in coming days. Please leave a comment if you have a question or you want us to post something specific.



16 Comments

  1. want to study how to solve aptitude test for all industries

    Reply
  2. How to solve this question? please answer me.
    The average of first five prime numbers is____.
    (1)5.40 (2)5.70 (3)5.60 (4)5.80

    Reply
    • average of first five prime no is 5.60
      sol:
      2+3+5+7+11/5 = 5.60

      Reply
  3. what will be the next for this series 9,19,40,69,149........

    Reply
  4. 0 12 =12 ;6*2=12
    10 3 =13 ;5*2=10
    6 8 =14 ;4*2=8
    6 9 =15 ;3*2=6
    12 4 =16 ;2*2=4

    Reply
  5. To Gabriel Baker. The answer is 81.
    They took the first number and added with 8, the next they added with 2 8's. and the next with 3 8's and so on.

    Reply
  6. I need help solving this number series
    0 12 10 3 6 8 6 9 __ __

    Please fill in the last two numbers and explain exactly how you got it. I am studying for a test. I do have a cheat sheet with the correct answers but i still am clueless on how they got the answer.

    Thank you for your help.

    Reply
  7. Hi,

    Please provide us few more sample questions for IOB - PO exam.It would be more helpful,if u posted the sample or previous questions papers.

    Thanks in Advance.

    Reply
  8. pls provide me solved more question and exam question

    Reply
  9. pls provide me more question and exam question

    Reply
  10. What would the next number be in this sequence: 1,9,25,49,_ ??

    Reply
    • Ans: 81
      1^2=1, 3^2=9, 5^2=25, 7^2=49, 9^2=81.

      Reply
    • To Gabriel Baker : 1,9,25,49,_??
      the Ans. is 81.
      (1)*(1)=1,(3)*(3)=9,(5)*(5)=25,(7)*(7)=49,(9)*(9)=81
      There are odd numbers square.

      Reply
      • To Gabriel Baker : 1,9,25,49,_??
        The answer is 81
        (1),(1+8=9),(9+8+8=25),(25+8+8+8=49),(49+8+8+8+8=81)
        In every case you have to add one additional 8 with the previous number.

    • 1+8=9,9+16=25,25+24=49,49+32=81,its just adding of table 8 to last no.

      Reply
  11. guide me with statement & assumptions in test of reasoning

    Reply

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