SSC CGL 2024 Tier-I · 25 September 2024, Shift 2 · General Intelligence and Reasoning · Question 1 of 99

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.

Select the number from among the given options that can replace the question mark (?) in the following series. 66, 75, 86, 99, 114, ?

(a)131
(b)140
(c)139
(d)135

Answer

Why

Correct — A. The gaps, not the terms, carry the pattern.

Rule: the first differences are 9, 11, 13, 15 — each one 2 more than the last.

75 − 66 = 9
86 − 75 = 11
99 − 86 = 13
114 − 99 = 15

Next gap = 15 + 2 = 17
114 + 17 = 131 → option (a)

Why the others are wrong

(b)
140 — 140 would need a jump of 26 from 114. The gaps here move 9, 11, 13, 15, so the next one is 17, and nothing in the run justifies 26.
(c)
139 — 139 leaves a gap of 25. That is the number you reach by adding the last gap and then a stray 10; the difference row only ever grows by 2.
(d)
135 — 135 leaves a gap of 21, which would mean the differences jumped by 6 in one step after rising by 2 four times running.

Concept

A series that resists a common ratio and a constant gap usually gives itself up on the first-difference row. Write the gaps underneath the terms and look at them on their own.

If that row is itself an arithmetic run, the series is a second-order one and the next gap is fixed — no guessing is required. Here the gap row is 9, 11, 13, 15, an arithmetic run with common difference 2, so the sixth gap has to be 17.

The four options sit within 9 of each other, so an approximate feel for the growth is not enough. Only the exact gap decides.

Key facts

The first differences of 66, 75, 86, 99, 114 are 9, 11, 13, 15.
Those differences rise by a constant 2, which makes this a second-order series.
The sixth term is 114 + 17 = 131.

Study next

Second-order number series and the difference table that solves themSeries whose gaps shrink by a constant instead of growingAlternating series, where two interleaved patterns run in one line of numbers

Common traps

Testing multiplication first, when terms this close together can only be additive
Getting the gap row right and then adding 15 again instead of 17
Adding the new gap to the wrong term

Plain number series turn up in the Reasoning block and the difference table is the first thing to try. The same method cracks 543, 518, 495, 474, 455, ? at 10 Sep 2024, 12:30, Reasoning Q.15, where the gaps shrink by 2 instead of growing.

Related PYQs

No directly related past PYQ was found.