SSC CGL 2024 Tier-I · 23 September 2024, Shift 2 · General Intelligence and Reasoning · Question 2 of 100

Text as printed on a candidate's response sheet for this shift; answer per SSC's tentative key as marked on that sheet. Solution last reviewed 10 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. The questions and options here are reproduced as printed on a candidate's own response sheet for this shift — the Commission's text, unchanged. Each answer is SSC's tentative key as marked on that sheet; SSC allowed challenges to it and never publishes a final key. The solutions are written by UnlockIAS.

Which of the following numbers will replace the question mark (?) in the given series? 81, 65, 50, ?, 23, 11

(a)37
(b)34
(c)35
(d)36

Answer

Why

Correct — D. Write the gaps between consecutive terms before trying anything else.

Rule: −16, −15, −14, −13, −12 — the subtraction shrinks by 1 each step.

81 − 16 = 65
65 − 15 = 50
50 − 14 = 36 → option (d)
36 − 13 = 23, and 23 − 12 = 11, which closes the series

Why the others are wrong

(a)
37 — 37 jumps to a gap of 13 a step too early. It would then need a drop of 14 to reach 23, so the shrinking gaps would start growing again.
(b)
34 — 34 repeats the opening gap of 16. It would then have to fall 11 to reach 23, skipping the 13 the pattern requires.
(c)
35 — 35 repeats the previous gap of 15. It also forces 35 to 23 to be a drop of 12, the same gap as 23 to 11, and no gap is used twice.

Concept

A number series with no obvious multiplier is almost always a difference series. Write the gaps under the terms first.

Here the gaps are 16, 15, 14, 13, 12 — each one smaller than the last by exactly 1. That second-level pattern is what is being tested, not the terms themselves.

The terms printed after the blank, 23 and 11, are a free check: whatever you insert must leave a gap of 13 down to 23 and 12 from 23 to 11.

Both ends of the series are supplied, so the blank can be reached by working forward from 50 or backward from 23. Doing both catches a mis-read gap before you commit.

Key facts

The gaps here are 16, 15, 14, 13, 12, falling by 1 each step.
A series whose gaps change by a constant is a second-difference series, meaning the differences of the differences are equal.
Terms printed after the blank are a free check on any candidate answer.

Study next

Series whose differences grow rather than shrink, such as +4, +8, +12Series built on alternating operations, where odd and even positions follow separate rulesMixed letter-number series, where a letter shift and a number gap run side by side

Common traps

Assuming a constant difference because 16 and 15 look close.
Working only forward from 50 and never testing the answer against 23.

A mixed letter-number series sits at Reasoning Q.16 of this shift — MY 84, KW 80, IU 76, GS 72 — where a steady letter shift and a steady number gap have to be tracked side by side.

Related PYQs

No directly related past PYQ was found.