UPSC Prelims 2017 · CSAT
Question 33: Logical Reasoning and Mental Ability
A clock strikes once at 1 o’clock, twice at 2 o’clock and thrice at 3 o’clock, and so on. If it takes 12 seconds to strike at 5 o’clock, what is the time taken by it to strike at 10 o’clock?
AnswerUPSC’s answer is (b).
The answer treats striking time as proportional to the number of strikes: 5 strikes take 12 seconds, so each strike takes 12 / 5 = 2.4 seconds. Ten strikes at 10 o'clock take 10 x 2.4 = 24 seconds. Counting the gaps between strikes instead (4 gaps for 5 strikes, so 3 seconds a gap) would give 9 gaps x 3 = 27 seconds, which is not among the options.
Why each option is right or wrong
- (a) 20 seconds does not follow from either method.
- (b) Correct: 12 s / 5 strikes = 2.4 s each, so 10 strikes = 24 s.
- (c) 28 seconds does not follow from either method.
- (d) 30 seconds does not follow from either method.
Worth knowing: Counting the gaps between strikes (4 gaps in 12 s, so 9 gaps in 27 s) gives 27 seconds, which is not an option; the key treats time as proportional to the number of strikes.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2017, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.