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27 Sep 2021, 07:14
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B
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
75%
(hard)
Question Stats:
61% (03:06) correct
39%
(02:11)
wrong
based on 28
sessions
History
Date
Time
Result
Not Attempted Yet
Alarm A rings once every 210 seconds
Alarm B rings once every 280 seconds
Alarm C rings once every 350 seconds
If they all ring simultaneously, we will need to wait for the LCM of 210, 280 and 350 seconds for them to all ring again. Since 70 is a common factor to all three numbers, we can find the LCM by ignoring that 70, and finding the LCM of 3, 4 and 5, which is 60, then multiplying by 70. So the LCM is 60*70 seconds, and dividing by 60, is 70 minutes. So 70 minutes later, they all ring again, and the time will be 7:10 PM.
The wording of the question isn't exactly clear -- it's not clear if we're looking for the time when they all start ringing simultaneously, or merely a time when the rings happen to overlap (even if they aren't all beginning simultaneously). I assume they meant the former interpretation above, because that's a much easier question to answer, though it turns out the answer is the same either way.
Alarm B rings once every 280 seconds
Alarm C rings once every 350 seconds
If they all ring simultaneously, we will need to wait for the LCM of 210, 280 and 350 seconds for them to all ring again. Since 70 is a common factor to all three numbers, we can find the LCM by ignoring that 70, and finding the LCM of 3, 4 and 5, which is 60, then multiplying by 70. So the LCM is 60*70 seconds, and dividing by 60, is 70 minutes. So 70 minutes later, they all ring again, and the time will be 7:10 PM.
The wording of the question isn't exactly clear -- it's not clear if we're looking for the time when they all start ringing simultaneously, or merely a time when the rings happen to overlap (even if they aren't all beginning simultaneously). I assume they meant the former interpretation above, because that's a much easier question to answer, though it turns out the answer is the same either way.
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27 Sep 2021, 11:07
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kapil1995
Total time taken by individual clocks equal 210 , 280 , 350 seconds respectively
Their LCm will help us in deciding on the overlapping time = 3*4*5 *7 *10 / 60*60 for the hour timing
=>7/6 =1+1/6 or rather after 1 hour and 10 min
Therefore IMO B
Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Problem Solving (PS) Forum
Still interested in this question? Check out the "Best Topics" block above for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
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