SSC CGL 2025 Tier-I · 15 September 2025, Shift 3 · General Intelligence and Reasoning · Question 21 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.

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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 group doesn’t match the pattern?

(a)20, 30, 50
(b)15, 25, 40
(c)18, 24, 36
(d)14, 28, 56

Answer

Why

Correct — B.
Rule: the second gap is double the first.

20, 30, 50: gaps 10 and 20
18, 24, 36: gaps 6 and 12
14, 28, 56: gaps 14 and 28
15, 25, 40: gaps 10 and 15, not 20 → option (b).

Why the others are wrong

(a)
20, 30, 50 — 20, 30, 50 has gaps of 10 then 20, the doubling the majority share. It also sums (20 + 30 = 50), but that rule fits 15, 25, 40 too, so it cannot pick the odd one.
(c)
18, 24, 36 — 18, 24, 36 steps 6 then 12, so its second gap doubles the first, like the others.
(d)
14, 28, 56 — 14, 28, 56 is a doubling series, which makes it look different. Its gaps, 14 then 28, still double, so it fits the shared rule.

Concept

In a three-number odd-one-out, test each rule against all four groups. The right rule fits exactly three.

First + second = third fits 20, 30, 50 and 15, 25, 40 but not the other two, a two-against-two split that decides nothing. Each term doubles fits only 14, 28, 56.

The gap rule fits three groups and leaves out 15, 25, 40.

A parity check agrees: 15, 25, 40 is the only group containing odd numbers. The gap rule is the stronger reason, because it ties the three numbers together.

Key facts

Second gap = 2 × first gap in 20, 30, 50, in 18, 24, 36 and in 14, 28, 56.
As a formula: third = 3 × second − 2 × first. For 15, 25, 40 it gives 75 − 30 = 45, not 40.
A rule that fits two groups and fails two cannot decide an odd-one-out.

Study next

Number triads linked by multiplication, such as first × second − thirdOdd-one-out on number properties: prime, square, cubeTriads whose gaps stand in a ratio other than 2

Common traps

Using first + second = third, which 15, 25, 40 also satisfies
Picking 14, 28, 56 because it is the one geometric group

12 Sep 2025, 09:00, Reasoning Q.18 sets the same three-number odd-one-out on a number property instead: 4, 6, 8 is keyed as the group that is not all prime.

Related PYQs

No directly related past PYQ was found.

Topic: Symbolic/Number Classification — all 45 SSC CGL questions on it, every solved shift.