Multiple choice

A faulty watch gains time uniformly. It is 9 minutes behind at noon on Sunday and 3 minutes 30 seconds fast at 7:00 P.M. on the following Sunday. When was it showing the correct time?

  1. Sunday, 5:00 P.M.

  2. Sunday, 6:00 P.M.

  3. Friday, 6:00 P.M.

  4. Friday, 5:00 P.M.

  5. Saturday, 6:00 P.M.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Total time elapsed from Sunday noon to next Sunday 7 PM = 7 days + 7 hours = 175 hours. Total gain = 9 min + 3.5 min = 12.5 min = 750 seconds. Rate of gain = 750/175 seconds/hour. To show correct time, it must gain 9 minutes (540 seconds). Time taken = 540 / (750/175) = 540 * 175 / 750 = 126 hours. 126 hours = 5 days and 6 hours. Sunday noon + 5 days 6 hours = Friday 6 PM.

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