What is the missing number ‘X’ of the series 7, X, 21, 31, 43 ?
- (a) 11
- (b) 12
- (c) 13 ✓ UPSC's answer
- (d) 14
Why the answer is (c)
• Try X = 13: the series becomes 7, 13, 21, 31, 43.
• The differences are 6, 8, 10, 12 — rising by 2 each time, a consistent pattern.
• Hence X = 13, option (c).
Why the other options are wrong
- (a) 11
- With X = 11 the differences are 4, 10, 10, 12 — no pattern.
- (b) 12
- With X = 12 the differences are 5, 9, 10, 12 — no pattern.
- (d) 14
- With X = 14 the differences are 7, 7, 10, 12 — no pattern.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2015, held on 23 August 2015. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.