Multiple choice

An antique clock which gains time uniformly is 10 minutes slow at 9:00 a.m. on Monday morning and 10 min 48 sec fast at 9:00 p.m. on the following Monday. When did it show the correct time during the intervening period?

  1. At 11:00 p.m. on Wednesday

  2. At 11:32 p.m. on Thursday

  3. At 11:40 p.m. on Wednesday

  4. At 11:28 p.m. on Thursday
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Total time from Monday 9 AM to next Monday 9 PM is 7 days + 12 hours = 180 hours. The clock gains 10 min + 10 min 48 sec = 20.8 minutes in 180 hours. It needs to gain 10 minutes to show the correct time. Time taken = (10 / 20.8) * 180 = 86.538 hours = 3 days 14 hours 32 minutes. Adding this to Monday 9 AM gives Thursday 11:32 PM.

AI explanation

The antique clock must gain 10 minutes plus 10 minutes and 48 seconds (a total of 20.8 minutes) to show the correct time. The total time elapsed is exactly 181 hours (7 days and 12 hours). By dividing 181 hours by 20.8 minutes, we find the clock gains 1 minute every 8.75 hours. It needs 87.5 hours to gain 10 minutes, and adding this to Monday at 9:00 a.m. brings the correct time to Thursday at 11:32 p.m.

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