SSC CGL 2025 Tier-I · 12 September 2025, Shift 3 · General Intelligence and Reasoning · Question 5 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.

Find the number that fits:2, 5, 10, 17, ?

(a)26
(b)24
(c)21
(d)20

Answer

Why

Correct — A.
Rule: n² + 1 for n = 1, 2, 3, 4, 5.

1² + 1 = 2, 2² + 1 = 5.
3² + 1 = 10, 4² + 1 = 17.
Next: 5² + 1 = 26.

Check with gaps: 3, 5, 7, so the next gap is 9.
17 + 9 = 26 → option (a).

Why the others are wrong

(b)
24 — 24 adds 7 again, repeating the last gap. The gaps climb through the odd numbers 3, 5, 7, so the next one is 9.
(c)
21 — 21 adds 4 to 17, an even gap. Every gap here is odd and 2 more than the one before, so after 7 comes 9.
(d)
20 — 20 adds 3, restarting the gaps from the beginning. They keep climbing, 3, 5, 7, so the next is 9, not 3.

Concept

Numbers one more than a perfect square are worth knowing on sight: 2, 5, 10, 17, 26, 37, 50.

The gap route gives the same answer. Consecutive squares differ by consecutive odd numbers (1, 4, 9, 16 have gaps 3, 5, 7), and adding 1 to every term leaves the gaps unchanged.

The series stops after four terms, so run both readings: squares plus 1, and gaps of 3, 5, 7. They agree on 26.

Key facts

n² + 1 for n = 1 to 7 gives 2, 5, 10, 17, 26, 37, 50.
Consecutive squares differ by consecutive odd numbers: 4 − 1 = 3, 9 − 4 = 5, 16 − 9 = 7.

Study next

Series of n² − 1 and n³ + 1Series where each term is the previous term squared plus 1Series of n × (n + 1)

Common traps

Repeating the last gap of 7 and choosing 24
Restarting the gaps at 3 and choosing 20

21 Sep 2025, 16:00, Reasoning Q.24 prints the same series one term longer, 2, 5, 10, 17, 26, and keys 37 = 6² + 1.

24 Sep 2024, 12:30, Reasoning Q.11 starts at 2² + 1: 5, 10, 17, 26, 37 is keyed 50.

Related PYQs

No directly related past PYQ was found.

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