2023 CSAT, all questions
UPSC Prelims 2023 · CSAT
Question 40: Basic Numeracy
Choose the group which is different from the others :
a17, 37, 47, 97
b31, 41, 53, 67
c71, 73, 79, 83
d83, 89, 91, 97
AnswerUPSC’s answer is (d).
Test each number for primality. 17, 37, 47 and 97 are all prime. 31, 41, 53 and 67 are all prime. 71, 73, 79 and 83 are all prime. In the last group, 83, 89 and 97 are prime but 91 = 7 x 13 is not. So group (d) is the odd one out. Remember 91 as the classic composite that looks prime.
Why each option is right or wrong
- (a) Wrong: all four numbers are prime.
- (b) Wrong: all four numbers are prime.
- (c) Wrong: all four numbers are prime.
- (d) Right: 91 = 7 x 13 is composite.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2023, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.