SSC CGL 2024 Tier-I · 13 September 2024, Shift 1 · General Intelligence and Reasoning · Question 4 of 100

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 10 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 among the given options that can replace the question mark (?) in the following series. 30, 20, ?, 60, 140

(a)30
(b)40
(c)35
(d)25

Answer

Why

Correct — A. Rule: multiply by 0.5, 1, 1.5, 2 and add 5, 10, 15, 20 — the multiplier grows by 0.5 and the addition by 5 at every step.

30 × 0.5 + 5 = 20
20 × 1 + 10 = 30
30 × 1.5 + 15 = 60
60 × 2 + 20 = 140

Every printed term is reproduced, and the gap takes 30, option (a).

A faster check: whatever fills the gap must satisfy ? × 1.5 + 15 = 60, so ? = 30.

Why the others are wrong

(b)
40 — 40 fails the step in front of it. 40 × 1.5 + 15 = 75, not the 60 the paper prints.
(c)
35 — 35 gives 35 × 1.5 + 15 = 67.5. That misses 60, and the series carries whole numbers throughout.
(d)
25 — 25 gives 25 × 1.5 + 15 = 52.5, short of 60 and again not a whole number.

Concept

When plain differences fail, test a multiply-and-add rule with both parts moving.

The differences here run −10, +10, +30, +80, which follow nothing. So look for a rule shaped as next = current × m + c, and let m and c each change by a fixed step.

Here m runs 0.5, 1, 1.5, 2 and c runs 5, 10, 15, 20. A series that falls and then climbs steeply is the signal for this family, because no constant difference and no constant ratio can do both.

The gap sits in the middle rather than at the end, so you can work backwards from the term after it. That is faster and safer than fixing a rule from the first two numbers alone.

Key facts

30 × 0.5 + 5 = 20 and 20 × 1 + 10 = 30 are the first two steps.
30 × 1.5 + 15 = 60 and 60 × 2 + 20 = 140 are the last two.
The multiplier rises by 0.5 each step and the added constant by 5.

Study next

Multiply-and-add series in which the multiplier and the constant both moveRecovering a missing middle term by working backwards from the term after itSeries that fall before they rise, which rule out a single ratio

Common traps

Hunting for a constant difference in a series that changes direction
Settling on a rule that fits the first two terms and never testing it on the last

SSC sets number series either as a next-term question or, as here, with the gap inside. Number relationships in a different dress also appear in this shift at Reasoning Q.6, Q.16, Q.20 and Q.21.

Related PYQs

No directly related past PYQ was found.