2020 CSAT, all questions
UPSC Prelims 2020 · CSAT
Question 13: Logical Reasoning and Mental Ability
A simple mathematical operation in each number of the sequence 14, 18, 20, 24, 30, 32, ... results in a sequence with respect to prime numbers. Which one of the following is the next number in the sequence?
a34
b36
c38
d40
AnswerUPSC’s answer is (c).
Subtract 1 from each term: 13, 17, 19, 23, 29, 31. These are consecutive prime numbers. The next prime after 31 is 37, so the next term is 37 + 1 = 38. Remember to try simple operations such as adding or subtracting 1 and see which one produces a recognisable series.
Why each option is right or wrong
- (a) 34 - 1 = 33, which is not prime (3 × 11).
- (b) 36 - 1 = 35, which is not prime (5 × 7).
- (c) Right: 37 is the next prime, so the term is 38.
- (d) 40 - 1 = 39, which is not prime (3 × 13).
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2020, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.