SSC CGL 2024 Tier-I · 12 September 2024, Shift 1 · General Intelligence and Reasoning · Question 15 of 99

Text as printed on a candidate's response sheet for this shift; answer per SSC's tentative key as marked on that sheet. Solution last reviewed 6 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. The questions and options here are reproduced as printed on a candidate's own response sheet for this shift — the Commission's text, unchanged. Each answer is SSC's tentative key as marked on that sheet; SSC allowed challenges to it and never publishes a final key. The solutions are written by UnlockIAS.

Select the number from the given options to complete the series. 25, 30, 40, 55, 75, ___

(a)80
(b)100
(c)105
(d)85

Answer

Why

Correct — B. Write the first differences under the terms.

30 − 25 = 5
40 − 30 = 10
55 − 40 = 15
75 − 55 = 20

Rule: +5, +10, +15, +20, … — each step is 5 more than the one before.

Next step = 20 + 5 = 25
75 + 25 = 100 → option (b)

Why the others are wrong

(a)
80 — 80 is 75 + 5, the first gap used again. The gaps have already grown to 20, so the next term cannot be only 5 away.
(c)
105 — 105 is 75 + 30, which makes the step jump from 20 to 30. The steps here grow by a constant 5, not by 10.
(d)
85 — 85 is 75 + 10, the second gap reused. Any repeat of an earlier gap ignores that the gaps themselves form the series 5, 10, 15, 20.

Concept

A series that resists a single common difference almost always yields to its first differences. Write the gaps underneath and read them as a series in their own right.

Here the gaps are 5, 10, 15, 20 — an arithmetic progression with common difference 5. When the differences are themselves in AP, the original series is quadratic, and you extend it by extending the gaps.

The check costs nothing: your answer minus the last printed term must equal the gap you predicted.

The terms also follow the closed form 25 + 5·(n−1)n/2, which is why the gaps climb by a constant 5: n = 6 gives 25 + 75 = 100.

Key facts

The first differences of 25, 30, 40, 55, 75 are 5, 10, 15, 20 — an AP with common difference 5.
When the first differences form an AP, the series itself is quadratic in n.
The sixth term is 75 + 25 = 100.

Study next

Second-difference series, where the gaps of the gaps are constantAlternating series, in which odd and even positions run separate rulesMultiply-and-add chains such as x2 + 1

Common traps

Adding the last gap again (75 + 20 = 95) instead of the next one.
Reading the gaps as doubling because 5 and 10 open the list.
Taking 105, which needs the step to leap from 20 straight to 30.

Reasoning Q.2 and Q.6 of this shift also ask you to read the gaps, but there the step is constant — +4 inside every cluster at Q.2, and a fixed step down each column at Q.6 — not the growing gap that decides this one.

Write the gaps down before you stare at the terms.

Related PYQs

No directly related past PYQ was found.