Questions on number series are very frequently asked in banking examinations and IBPS RRB Office Assistant Prelims is no exception. These questions are easy to answer, takes comparatively less time and are scoring. You should memorize squares of numbers up to 30 and cubes of numbers up to 20 to solve these questions quickly.

In these questions, a series is given where numbers are placed next to each other with a logic. Your job is to find out that logic and answer the question accordingly. Generally, three kinds of questions can be asked:

  1. A number in the series is missing which you need to find out.
  2. A number in the series is placed incorrectly. You need to find out the incorrect number.
  3. A complete numerical series is followed by an incomplete numerical series. You need to complete the latter one.

Whatever kind of question is asked, if your basics are strong you can easily deal with it.

Things you need to keep in mind

  • If the number series has an increasing trend, the multiplication and/or addition operators would have been used.
  • On the contrary, if it has a decreasing trend, the division and/or subtraction operators would have been used.
  • If the series shows sudden surge/decline, square/cube/square root/cube root might be in the play.
  • If the trend is uneven (i.e. increasing and then decreasing and so on), there would be a combination of any of the above factors at work.

One thing is for sure that there isn’t any set of formula that would work for all the questions. You need to resort to ‘hit and trial’ methodology and keep trying different combinations unless you achieve success. And that’s why practicing beforehand becomes very important for this topic. When you solve different questions, you get comfortable using a combination of different concepts which eventually helps you in the actual exam. Practicing via a number of mock tests actually helps you achieve this expertise. In case you are unable to solve the question or solved the question incorrectly, the detailed solution provided in the mock tests helps you understand your shortcoming.

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Some questions asked last year

  1. Find the missing number in the given series. 6, 8, 13, 23, ?, 56
    1. 32
    2. 40
    3. 44
    4. 46
    5. None of these
  2. Find the missing number in the given series. 7, 8, 18, 57, 232, ?
    1. 1,165
    2. 1,100
    3. 1,160
    4. 1,210
    5. None of these
  3. Find the missing number in the given series. 8, 5, 6, 10, 21, ?
    1. 50.5
    2. 52.25
    3. 55
    4. 53.5
    5. None of these

Explanation of the first question

The differences between the numbers in the series are 2, 5 and 10. On calculating the difference between these numbers the differences come out to be 3 & 5 which shows odd numbers on an increasing pattern. The next odd number would be 7 which when added to 10 will become 17. This is also explained in the below figure:

Explanation of the second question

The series is moving rapidly ahead. So the difference won’t gives us much and the rapid spurt suggests us that there may be multiplication into play. On giving a try, multiplying the first number with 1 and adding 1 to it gets us to the second number. The third number is obtained by multiplying the second number with 2 and adding 2 afterward and so on. Therefore, 232 when multiplied by 5 is 1,160 and 5 added to it makes it 1,165. Please review the below figure as well.

Explanation of the third question

Taking out the difference suggests us that the series declines initially and then goes high. On trial and error, it can be seen that each number when multiplied by 0.5 and 1 added thereafter gives us the next number as shown in the figure below.


It’s obvious that the first 2 questions are easy enough to be solved. The third question is a little tricky since the multiplication of decimal is coming into the picture. If these types of questions get solved within a minute it’s fine otherwise you should not spend more than 1 minute on these types of questions. Once you get a hands-on experience by practicing, solving questions on number series shouldn’t be that difficult for you.

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