2017 CSAT, all questions

UPSC Prelims 2017 · CSAT

Question 31: Logical Reasoning and Mental Ability

A watch loses 2 minutes in every 24 hours while another watch gains 2 minutes in every 24 hours. At a particular instant, the two watches showed an identical time. Which of the following statements is correct if 24-hour clock is followed?

aThe two watches show the identical time again on completion of 30 days.
bThe two watches show the identical time again on completion of 90 days.
cThe two watches show the identical time again on completion of 120 days.
dNone of the above statements is correct.

AnswerUPSC’s answer is (d).

One watch loses 2 minutes a day and the other gains 2 minutes, so they drift apart by 4 minutes each day. On a 24-hour clock they show the same time again only when the gap is a full 24 hours, which is 1440 minutes. That takes 1440 / 4 = 360 days. This is not 30, 90 or 120 days, so none of the given statements is correct. On a 12-hour clock the answer would be 180 days.

Why each option is right or wrong
  • (a) After 30 days the gap is only 120 minutes.
  • (b) After 90 days the gap is only 360 minutes.
  • (c) After 120 days the gap is 480 minutes, just 8 hours.
  • (d) Correct: they meet again after 360 days.

Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2017, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.