SSC CGL 2025 Tier-I · 18 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 13 September 2026.

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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.

Which of the following groups does not follow the same arithmetic rule?

(a)10, 20, 40
(b)5, 15, 45
(c)2, 6, 18
(d)3, 9, 27

Answer

Why

Correct — A.
Rule: each number is 3 times the one before it (×3, ×3).

5 → 15 → 45: ×3, ×3 ✓
2 → 6 → 18: ×3, ×3 ✓
3 → 9 → 27: ×3, ×3 ✓

10 → 20 → 40: ×2, ×2 ✗

So 10, 20, 40 is the group that breaks the rule → option (a).

Why the others are wrong

(b)
5, 15, 45 — 5, 15, 45 follows the rule: 5 × 3 = 15 and 15 × 3 = 45. It belongs with 2, 6, 18 and 3, 9, 27.
(c)
2, 6, 18 — 2, 6, 18 multiplies by 3 at each step, 2 × 3 = 6 and 6 × 3 = 18, the rule the majority shares.
(d)
3, 9, 27 — 3, 9, 27 stands out as powers of 3, but that is still ×3 at each step, the same rule as 5, 15, 45 and 2, 6, 18.

Concept

In a group odd-one-out, test a relation inside each group, not the numbers themselves. Here each group is a chain of equal ratios: the second number divided by the first, and the third by the second.

Three groups have ratio 3, one has ratio 2. The group with the different ratio is the one that does not follow the shared rule.

Differences say the same thing. In 5, 15, 45 the gaps are 10 and 30, the second 3 times the first. In 10, 20, 40 the gaps are 10 and 20, only double.

Key facts

A group a, b, c follows a ×k rule when b = k × a and c = k × b.
10, 20, 40 has ratio 2.
Options (b), (c) and (d) all have ratio 3.

Study next

Groups built on squares and cubes (n, n², n³)Groups where the third number is the sum or product of the first twoWrong-number series built on a ×3 chain

Common traps

Marking 3, 9, 27 as odd because it is a set of powers of 3: it still moves ×3 like (b) and (c)
Comparing differences without ratios: 10 → 20 and 5 → 15 both add 10, which hides the pattern

24 Sep 2025, 16:00, Reasoning Q.16 runs the same ×3 chain, 2, 6, 18, 54, 162, and keys 325 as the number that does not belong (162 × 3 = 486).

13 Sep 2025, 12:30, Reasoning Q.17 uses triads with a different rule, n, n², n³: 2, 4, 8 and 3, 9, 27 fit there, and 4, 16, 36 is keyed.

Related PYQs

No directly related past PYQ was found.

Topic: Symbolic/Number Classification — all 50 SSC CGL questions on it, every solved shift.