SSC CGL 2025 Tier-I · 12 September 2025, Shift 2 · 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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Find the number that breaks the pattern: 2, 5, 10, 17, 28, 40, 58

(a)10
(b)17
(c)40
(d)28

Answer

Why

Correct — C. Take the gaps between terms.

Rule: add consecutive primes, 3, 5, 7, 11, 13, 17.

2 + 3 = 5
5 + 5 = 10
10 + 7 = 17
17 + 11 = 28

28 + 13 = 41, not 40
41 + 17 = 58, which the series does print.

So 40, option (c), breaks the pattern: 41 repairs both gaps around it.

Why the others are wrong

(a)
10 — 10 = 5 + 5, the second prime step. Both gaps around it, 5 and 7, are consecutive primes, so it fits.
(b)
17 — 17 = 10 + 7, and the next gap, 11, is the next prime. Nothing around 17 is out of step.
(d)
28 — 28 = 17 + 11 fits. The faulty gap is 28 to 40, which is 12, and it is 40 that must change: 41 gives 13, then 17 more reaches 58.

Concept

In a 'breaks the pattern' series, ask which single change makes every gap fit.

The terms here are running sums of primes: 2, 2 + 3, 2 + 3 + 5, and so on, so the gaps are the primes from 3 up. A wrong term in the middle spoils two gaps side by side, 12 and 18 here, and the term between them is the culprit.

The first four terms also fit n² + 1 (2, 5, 10, 17), whose gaps are the odd numbers 3, 5, 7, 9.

That rule predicts 26, 37, 50 and would make three terms wrong, so it cannot be the pattern. The fourth gap, 11 rather than 9, is where primes and odd numbers part.

Key facts

Primes from 3: 3, 5, 7, 11, 13, 17.
Running sums of primes: 2, 5, 10, 17, 28, 41, 58.
One wrong middle term spoils the gaps on both sides of it.

Study next

Series whose gaps are primes or squaresWrong-number series built on factorialsRunning-sum (cumulative) series

Common traps

Seeing 2, 5, 10, 17 as n² + 1 and expecting 26 next.
Blaming 28 because 28 → 40 is the first bad gap, when 28 is forced by 17 + 11.

Wrong-number series are also set at 21 Sep 2025, 16:00, Reasoning Q.6, where 5040, 720, 120, 24, 4, 2 are the factorials 7! down to 2! with 4 in place of 6.

At 19 Sep 2025, 09:00, Reasoning Q.6, the gaps in 336, 246, 166, 94, 36, −14 should fall by 10, so 94 should be 96.

Related PYQs

No directly related past PYQ was found.

Topic: Number Series — all 178 SSC CGL questions on it, every solved shift.