A clock is 3 minutes slow at 9:00 AM and 7 minutes fast at 3:00 PM. At what time (in AM) will it show the correct time?
Show answer & explanation
Correct answer: B
Explanation
At 9:00 AM, the clock is 3 minutes slow.
At 3:00 PM, the clock is 7 minutes fast.
That is, between 9:00 and 3:00 (6 hours = 360 minutes), the clock's error:
went from −3 minutes to +7 minutes.
Total change = 10 minutes
That is, the clock's error increases every minute:
10/360 = 1/36 minute
The clock must make up 3 minutes to become correct.
Time required:
3 × 36 = 108 minutes
108 minutes = 1 hour 48 minutes
Therefore,
9:00 + 1 hour 48 minutes = 10:48 AM