SSC CGL 2024 Tier-I · 18 September 2024, Shift 2 · General Intelligence and Reasoning · Question 14 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.

Which of the following numbers will replace the question mark (?) in the given series? 20, 25, 25, 35, 30, 45, ?, 55, 40

(a)60
(b)35
(c)50
(d)30

Answer

Why

Correct — B. Two runs are interleaved here, so read the alternate terms separately.

Rule: odd places rise by 5, even places rise by 10.

Odd places (1st, 3rd, 5th, 7th, 9th): 20, 25, 30, ?, 40 → +5 each
Even places (2nd, 4th, 6th, 8th): 25, 35, 45, 55 → +10 each

The question mark sits at the 7th place, an odd place, so it belongs to the +5 run: 30 + 5 = 35.

The last term, 40, checks it — 35 + 5 = 40, which is what the paper prints. That is option (b).

Why the others are wrong

(a)
60 — 60 overshoots. The odd-place run reads 20, 25, 30, ?, 40, and a term sitting between 30 and 40 cannot be 60.
(c)
50 — 50 is what you get by averaging the neighbours 45 and 55, which treats the row as a single run. The 7th term belongs to the odd-place run climbing in fives.
(d)
30 — 30 repeats the term already used at the 5th place. An arithmetic run adding 5 has to move on, and its next value is 35.

Concept

An alternating series hides two independent runs inside one row. The 1st, 3rd, 5th … terms belong to one run and the 2nd, 4th, 6th … to the other, and neither run is disturbed by the other.

The tell is a row that will not settle into a single difference. Here 20 → 25 is +5, 25 → 25 is 0 and 25 → 35 is +10, so no one rule survives three steps.

Split the row and both halves become plain arithmetic progressions: +5 on the odd places, +10 on the even places. Nothing harder than that is needed.

Count the places before you compute. The question mark is the 7th of nine terms, and its place index alone decides which of the two runs governs it.

Key facts

The odd-place terms are 20, 25, 30, 35, 40, rising by 5 each time.
The even-place terms are 25, 35, 45, 55, rising by 10 each time.
The question mark occupies the 7th place, so the +5 run fixes it at 35.
The final term 40 confirms the answer, since 35 + 5 = 40.

Study next

Alternating number series built from two interleaved arithmetic runsAlternating series where one run adds and the other multipliesFixing a term's place index before choosing which run governs it

Common traps

Treating the row as one series and hunting for a single difference that fits every step.
Miscounting the question mark as the 6th term, which hands it to the even-place run.
Reading the repeated 25 as a stall in the series rather than as two runs crossing.

The stem names what it wants — which of the following numbers will replace the question mark — and all the work is in the arithmetic underneath.

23 Sep 2024, 12:30, Reasoning Q.2 runs 81, 65, 50, ?, 23, 11 on a single shrinking difference, and 25 Sep 2024, 16:00, Reasoning Q.15 runs 88, 78, 70, 64, 60, ?, ? the same way, with two blanks instead of one. Test the alternating split as soon as one difference fails to fit.

Related PYQs

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