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Question

Select the alphanumeric-cluster that can replace the question mark (?) in the following series? 

S-81, Q-64, ?, M-36, K-25

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
O-49

Alphanumeric Series Pattern Analysis

The given series is S-81, Q-64, ?, M-36, K-25. We need to find the missing alphanumeric cluster. The series can be broken down into two independent patterns: one for the letters and one for the numbers.

Letter Sequence Decoding

Examine the letters: S, Q, ?, M, K.

  • Convert letters to their alphabetical positions: S (19), Q (17), M (13), K (11).
  • The numerical sequence of positions is 19, 17, ?, 13, 11.
  • This is an arithmetic progression decreasing by 2: $19 - 2 = 17$, $17 - 2 = 15$, $15 - 2 = 13$, $13 - 2 = 11$.
  • The missing position is 15.
  • The 15th letter of the alphabet is O.

Number Sequence Decoding

Examine the numbers: 81, 64, ?, 36, 25.

  • These numbers are perfect squares. Find their square roots: $\sqrt{81} = 9$, $\sqrt{64} = 8$, $\sqrt{36} = 6$, $\sqrt{25} = 5$.
  • The sequence of square roots is 9, 8, ?, 6, 5.
  • This is an arithmetic progression decreasing by 1: $9 - 1 = 8$, $8 - 1 = 7$, $7 - 1 = 6$, $6 - 1 = 5$.
  • The missing square root is 7.
  • Therefore, the missing number is $7^2 = 49$.

Completing the Series Cluster

Combine the decoded letter (O) and number (49) to form the missing cluster.

  • The missing alphanumeric cluster is O-49.
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