SSC CGL 2024 Tier-I · 17 September 2024, Shift 3 · General Intelligence and Reasoning · Question 21 of 98

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. 1225, 1184, 1147, 1116, 1087, ?

(a)1078
(b)1072
(c)1054
(d)1064

Answer

Why

Correct — D. Write the differences under the terms.

1225 → 1184 is −41
1184 → 1147 is −37
1147 → 1116 is −31
1116 → 1087 is −29

Rule: the amounts subtracted are 41, 37, 31, 29 — consecutive primes, taken downwards.

The next prime below 29 is 23.

1087 − 23 = 1064 → option (d).

Why the others are wrong

(a)
1078 — 1078 needs a gap of 9, and 9 is not prime — it is 3 × 3. It also skips 23, 19, 17, 13 and 11 to get there.
(b)
1072 — 1072 needs a gap of 15, which is 3 × 5. Every gap in this series is prime, and the prime that follows 29 downwards is 23.
(c)
1054 — 1054 needs a gap of 33, which is 3 × 11. It is also larger than the previous gap of 29, while these gaps shrink at every step.

Concept

When a number series falls by uneven amounts, the differences are the real series. Write them out before anything else.

Differences that are not a tidy arithmetic run are often primes, squares or cubes. Check primes first — the list is short enough to recognise on sight.

Here the differences are 41, 37, 31 and 29, which is the prime list read backwards from 41, so the next term subtracts the next prime down.

41 and 37 sit close together and so do 31 and 29, with a wider gap between the pairs. That is why the differences look irregular until you name them as primes.

Key facts

The primes running down from 41 are 41, 37, 31, 29, 23 and 19.
The successive differences here are −41, −37, −31 and −29.
1087 − 23 = 1064.

Study next

Recognising the primes below 100 on sight so a difference row can be matched instantlySeries whose differences are squares or cubes rather than primesSeries that run the primes upwards, where the gaps widen instead of closing

Common traps

Hunting for a second-level difference, which here is 4, 6, 2 and leads nowhere.
Subtracting the next prime from 1116 rather than from 1087.
Treating 27, 33 or 9 as prime while scanning for the next gap.

Write the difference row under the terms first. If it is not an arithmetic run, test it against the primes and then the squares before looking for anything more elaborate.

Related PYQs

No directly related past PYQ was found.