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Question

What will come in place of question mark (?) in the following number series?

2, 5, 11, 23, 44, 77, ?

This question was previously asked in
RBI Assistant Prelims Memory Based Paper (27 March 2022) (Shift 2)
The correct answer is

125

Number Series Pattern Analysis: 2, 5, 11, 23, 44, 77, ?

To find the missing number in the series 2, 5, 11, 23, 44, 77, ?, we need to identify the underlying pattern. Let's examine the differences between consecutive terms:

  • $5 - 2 = 3$
  • $11 - 5 = 6$
  • $23 - 11 = 12$
  • $44 - 23 = 21$
  • $77 - 44 = 33$

The sequence of differences is: 3, 6, 12, 21, 33.

Now, let's look at the differences between these differences (the second-order differences):

  • $6 - 3 = 3$
  • $12 - 6 = 6$
  • $21 - 12 = 9$
  • $33 - 21 = 12$

The second-order differences form a sequence: 3, 6, 9, 12.

We can observe that this sequence (3, 6, 9, 12) is an arithmetic progression with a common difference of 3. The next term in this arithmetic progression would be:

$12 + 3 = 15$

Now, we can find the next difference in the first sequence of differences by adding this value (15) to the last difference (33):

$33 + 15 = 48$

Finally, to find the missing number in the original series, we add this calculated difference (48) to the last term of the original series (77):

$77 + 48 = 125$

Therefore, the number that comes in place of the question mark (?) is 125.

Series Analysis Summary
Term Number 1st Difference 2nd Difference 3rd Difference
1 2
2 5 $5 - 2 = 3$
3 11 $11 - 5 = 6$ $6 - 3 = 3$
4 23 $23 - 11 = 12$ $12 - 6 = 6$ $6 - 3 = 3$
5 44 $44 - 23 = 21$ $21 - 12 = 9$ $9 - 6 = 3$
6 77 $77 - 44 = 33$ $33 - 21 = 12$ $12 - 9 = 3$
7 ? $33 + (12 + 3) = 48$ $12 + 3 = 15$ $3$
Result $77 + 48 = 125$
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