SSC CGL 2024 Tier-I · 18 September 2024, Shift 1 · General Intelligence and Reasoning · Question 23 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 among the given options that can replace the question mark (?) in the following series. 300, 152, 80, 48, 40, 52, ?

(a)60
(b)100
(c)80
(d)90

Answer

Why

Correct — D. The first differences run −148, −72, −32, −8, +12. They never settle, so the rule is not a difference at all.

Rule: halve the term, then add the next power of 2 (+2, +4, +8, +16, +32, +64 …).

300 ÷ 2 = 150, + 2 = 152
152 ÷ 2 = 76, + 4 = 80
80 ÷ 2 = 40, + 8 = 48

48 ÷ 2 = 24, + 16 = 40
40 ÷ 2 = 20, + 32 = 52

The added power doubles once more, to 64.
52 ÷ 2 = 26, + 64 = 90 — option (d).

Why the others are wrong

(a)
60 — 60 would need 26 + 34, and 34 is not a power of 2. The additions have run 2, 4, 8, 16, 32, so the next one is fixed at 64.
(b)
100 — 100 would need 26 + 74. The series adds a doubling power of 2 at each step and 74 is not one of them.
(c)
80 — 80 repeats the third term, as though the series turned round at its lowest value and retraced itself. It would need 26 + 54, which breaks the doubling.

Concept

When first differences refuse to settle, the series is usually built by an operation on each term rather than by an addition to it. Halving is the first thing to try when terms fall steeply and then flatten: 300, 152, 80, 48 are each roughly half the term before.

The constant added afterwards is what turns the series upward. Once the added power of 2 grows past half the current term, the terms start to rise again, which is exactly what happens between 40 and 52.

A series that falls and then rises is a signal that the rule has two parts pulling in opposite directions. Checking whether each term is near half the one before takes a few seconds and rules that shape in or out.

Key facts

The rule is term ÷ 2 plus a power of 2, with the power doubling at each step: 2, 4, 8, 16, 32, 64.
The series turns upward at the sixth term because the added 32 is larger than half of 40.
First differences do not solve this series — they run −148, −72, −32, −8, +12.

Study next

Series built on term ÷ 2 plus a changing constantSeries where the added term is a power of 2, a square or a cubeChoosing between difference-based and operation-based series from the shape of the terms

Common traps

Hunting through first and second differences when the rule is multiplicative.
Adding 64 to 52 without halving first, which gives 116.
Assuming the series is symmetric about its lowest term and answering 80.

SSC sets a series whose terms fall and then rise, which is the standard signal for a two-part rule. The options are clustered near the true value — 60, 80, 90, 100 — so the rule has to be derived rather than estimated.

Related PYQs

No directly related past PYQ was found.