UPSC Prelims 2021 · CSAT
Question 26: Logical Reasoning and Mental Ability
Replace the incorrect term by the correct term in the given sequence 3, 2, 7, 4, 13, 10, 21, 18, 31, 28, 43, 40 where odd terms and even terms follow the same pattern.
AnswerUPSC’s answer is (a).
Take the odd-position terms: 3, 7, 13, 21, 31, 43. Their differences are 4, 6, 8, 10, 12, rising by 2 each time. The even-position terms should follow the same pattern: 2, 4, 10, 18, 28, 40 have differences 2, 6, 8, 10, 12, so the first difference should be 4, not 2. Replacing the first even term, 2, by 0 gives differences 4, 6, 8, 10, 12. The incorrect term is 2 and the correct term is 0. Remember: split interleaved sequences, then compare differences.
Why each option is right or wrong
- (a) Right: 0 makes the even terms 0, 4, 10, 18, 28, 40 with differences 4, 6, 8, 10, 12.
- (b) 1 would give a first difference of 3, which breaks the pattern.
- (c) 3 would give a first difference of 1, which breaks the pattern.
- (d) 6 would make the first difference −2 instead of 4.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2021, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.