2019 CSAT, all questions
UPSC Prelims 2019 · CSAT
Question 48: Logical Reasoning and Mental Ability
What is X in the sequence 132, 129, 124, 117, 106, 93, X ?
a74
b75
c76
d77
AnswerUPSC’s answer is (c).
The gaps between successive terms are 3, 5, 7, 11 and 13: the odd primes in order (132 to 129, 129 to 124, 124 to 117, 117 to 106, 106 to 93). The next odd prime is 17, so X = 93 - 17 = 76. Note that 9 is skipped in the gaps because it is not prime.
Why each option is right or wrong
- (a) Wrong: 74 would subtract 19, skipping 17.
- (b) Wrong: 75 would subtract 18, which is not prime.
- (c) Right: 93 - 17 = 76.
- (d) Wrong: 77 would subtract 16, which is not prime.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2019, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.