UPSC Prelims 2019 · CSAT
Question 49: Logical Reasoning and Mental Ability
A wall clock moves 10 minutes fast in every 24 hours. The clock was set right to show the correct time at 8:00 a.m. on Monday. When the clock shows the time 6:00 p.m. on Wednesday, what is the correct time ?
AnswerUPSC’s answer is (a).
The clock gains 10 minutes in each 24 hours of real time, so 24 hours of real time show as 24 h 10 min on the clock, that is 1450 minutes per 1440. From 8:00 a.m. Monday to 6:00 p.m. Wednesday the clock has run 58 h = 3480 min. Real time = 3480 × 1440 / 1450 = 3456 min = 57 h 36 min. Adding this to 8:00 a.m. Monday gives 5:36 p.m. Wednesday.
Why each option is right or wrong
- (a) Right: 3456 real minutes after 8:00 a.m. Monday is 5:36 p.m.
- (b) Wrong: that would be a 30-minute gain, but the clock gains about 24 minutes.
- (c) Wrong: 5:24 p.m. subtracts a gain of 36 minutes.
- (d) Wrong: 5:18 p.m. subtracts a gain of 42 minutes.
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.