SSC CGL 2025 Tier-I · 18 September 2025, Shift 2 · General Intelligence and Reasoning · Question 23 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.

Where this paper and its answers come from

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.

Find the next number in the series: 10, 21, 43, 87, ?, 351

(a)175
(b)173
(c)177
(d)179

Answer

Why

Correct — A.
Rule: double the term, then add 1 (× 2 + 1).

10 × 2 + 1 = 21
21 × 2 + 1 = 43
43 × 2 + 1 = 87
87 × 2 + 1 = 175

Check against the last term: 175 × 2 + 1 = 351 ✓ → option (a).

Why the others are wrong

(b)
173 — 173 is 87 × 2 − 1. The rule adds 1, and 173 × 2 + 1 = 347 misses the printed 351.
(c)
177 — 177 leads to 177 × 2 + 1 = 355 for the next term, four more than the printed 351.
(d)
179 — 179 leads to 179 × 2 + 1 = 359, not 351. It is also 4 more than 87 × 2 + 1 = 175.

Concept

When each term is roughly double the one before, test × 2 with a small correction. Here 10 → 21 is × 2 + 1, and the same step holds all the way along.

The gaps confirm it: 11, 22, 44, 88, 176, each double the last. 87 + 88 = 175, and 175 + 176 = 351, so the blank is fixed from both sides.

A term after the blank, like 351 here, is a free check: whatever fills the blank must lead to it under the same rule. Working backwards, (351 − 1) ÷ 2 = 175.

Key facts

In a × 2 + 1 series the gaps double at each step.
Gaps here: 11, 22, 44, 88, 176.
Backwards, the step is (term − 1) ÷ 2.

Study next

× n + n series (× 1 + 1, × 2 + 2, × 3 + 3)× 2 + 2 series such as 2, 6, 14, 30, 62Wrong-term series built on × 2 plus a constant

Common traps

Using − 1 instead of + 1 at the last step, which gives 173
Ignoring 351, the term after the blank that confirms the answer

17 Sep 2025, 16:00, Reasoning Q.24 uses the same × 2 + 1 rule on 3, 7, 15, 31, 63, keyed 127.

19 Sep 2025, 09:00, Reasoning Q.24 does it on 2, 5, 11, 23, 47, keyed 95.

Related PYQs

No directly related past PYQ was found.

Topic: Number Series — all 112 SSC CGL questions on it, every solved shift.