SSC CGL 2025 Tier-I · 14 September 2025, Shift 1 · General Intelligence and Reasoning · Question 18 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 missing number: 1, 4, 9, 16, ?, 36

(a)20
(b)25
(c)30
(d)27

Answer

Why

Correct — B. Each printed term is a perfect square.

Rule: the nth term is n².

1 = 1², 4 = 2², 9 = 3², 16 = 4²
Missing term = 5² = 25
Next term = 6² = 36, as printed

The differences agree: 3, 5, 7, 9, 11, consecutive odd numbers. That is option (b).

Why the others are wrong

(a)
20 — 20 is not a perfect square. It would make the gaps 16 → 20 → 36 equal to 4 and 16, breaking the 3, 5, 7 run, whose next gap is 9.
(c)
30 — 30 is not a perfect square: 5² = 25 and 6² = 36. It would also leave a gap of just 6 before the printed 36.
(d)
27 — 27 is 3³, a cube, not a square. From 16 it is a jump of 11, which breaks the 3, 5, 7 run of odd differences.

Concept

Before hunting for a difference pattern, check whether the terms are familiar powers. 1, 4, 9, 16 and 36 are the squares of 1, 2, 3, 4 and 6, so the gap is 5².

The two readings agree because consecutive squares differ by consecutive odd numbers: (n + 1)² − n² = 2n + 1.

The blank sits between two printed terms, 16 and 36, so an answer must fit both sides. 25 is 9 more than 16 and 11 less than 36, continuing the odd differences.

Key facts

Squares from 1² to 6²: 1, 4, 9, 16, 25, 36.
Consecutive squares differ by consecutive odd numbers, since (n + 1)² − n² = 2n + 1.
27 = 3³ is a cube, not a square.

Study next

Squares with a constant added, such as n² + 1Cube series and n³ − 1 patternsSeries built on n × (n + 1), such as 2, 6, 12, 20

Common traps

Looking for a constant difference, when the gaps 3, 5, 7 grow by 2 each time.
Choosing 27 because it is a familiar power, though it is a cube.

Squares with 1 added decide 21 Sep 2025, 16:00, Reasoning Q.24, where 2, 5, 10, 17, 26 continue to 37.

15 Sep 2025, 12:30, Reasoning Q.7 is built on powers as well: 1, 4, 27, 256 are 1¹, 2², 3³, 4⁴, so 5⁵ = 3125 comes next.

Related PYQs

No directly related past PYQ was found.

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