SSC CGL 2025 Tier-I · 14 September 2025, Shift 2 · General Intelligence and Reasoning · Question 12 of 100

We transcribed this text from the pictures on a candidate's response sheet for this shift; the answer is the key SSC colour-marked on that sheet. Solution last reviewed 11 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. From 2025 the candidate response sheet prints every question and option as a picture, so the text here is transcribed from those pictures for this shift and checked against them independently; where the layout carries meaning, the picture is shown as well. Each answer is the key SSC colour-marked on that sheet. SSC released a tentative key for challenge and later its final one, both only to candidates behind a login, and the sheet does not say which of the two it carries. The solutions are written by UnlockIAS.

Complete the series. 11, 13, 17, 19, ?

(a)21
(b)23
(c)25
(d)27

Answer

Why

Correct — B.
Rule: the prime numbers in order: 11, 13, 17, 19.

The numbers in between are skipped because they have factors: 15 = 3 × 5, and 12, 14, 16, 18 are even.

After 19: 20 = 2 × 10, 21 = 3 × 7, 22 = 2 × 11. The next prime is 23 → option (b).

Why the others are wrong

(a)
21 — 21 = 3 × 7, so it is not prime. It would also need a gap of +2 after 19, where the gaps alternate +2, +4.
(c)
25 — 25 = 5 × 5, a square, not a prime. A gap of +6 after 19 fits neither the prime list nor the +2, +4 rhythm.
(d)
27 — 27 = 3 × 3 × 3, a cube, not a prime. It would need a gap of +8 after 19.

Concept

When a short series has small, uneven gaps and odd terms, test for primes: numbers with no factor other than 1 and themselves. From 11 the primes run 11, 13, 17, 19, 23, 29.

A second reading also works here: the gaps alternate +2, +4, +2, so the next is +4, giving 23. Both readings agree, which makes 23 safe.

Key facts

The primes between 10 and 30 are 11, 13, 17, 19, 23 and 29.
21 = 3 × 7, 25 = 5² and 27 = 3³: none is prime.
To test a number for primality, divide by the primes up to its square root.

Study next

Odd-one-out items built on prime numbersSeries of powers, such as 1, 4, 27, 256 (1¹, 2², 3³, 4⁴)Divisibility tests for 3, 7 and 11

Common traps

Continuing with a +2 gap and choosing 21, which is 3 × 7

12 Sep 2025, 09:00, Reasoning Q.17 turns the same knowledge into an odd-one-out: 2, 3, 5, 7, 11, 13, 17, 20, keyed 20.

12 Sep 2025, 09:00, Reasoning Q.18 asks it of groups of three, keyed 4, 6, 8 against groups of primes.

Related PYQs

No directly related past PYQ was found.

Topic: Number Series — all 112 SSC CGL questions on it, every solved shift.