You have the misfortune to own an unreliable clock. This one loses exactly 20 minutes every hour. It is now showing 4:00am and you know that is was correct at midnight, when you set it. The clock stopped 4 hours ago, what is the correct time now?
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14 comments:
4PM
For each of the four hours the clock worked it lost 20 minutes meaning that when it stopped at 4:00 it had been going for an hour and 20 minutes for each time the minute hand passed 12. So, when it stopped one can add 4 times 20 minutes to the actual time, meaning the time was 5:10 at that point. Since it has been 4 hours since the clock stopped the actual time is 9:20
9:20 am
10 am
5:20 A.M. since it loses 20 minutes on every hour, it must have lost 80 minutes in 4 hours it's showing.
The correct answer is 10:00am.
The clock loses 20 minutes every hour or 1 hour every 3 hours. At 3:00am the clock would read 2:00am; at 6:00am the clock would read 4:00am. The clock stopped at 6:00am four hours ago. The time now is 10:00am.
The clock loses 20 minutes per hour. Therefore it loses one hour every three hours of real time. Thus it will show 2am when it is really 3 am, and it will show 4 am when it is really 6am. Thus it was 6 am when the clock stopped. It stopped 4 hours ago, so the time now is...
10am.
Now, let's look at it in terms of the misfortune clock as some of the other commenters have. The clock loses 20 minutes in every hour of REAL time. That means that it has lost 20 minutes when it is showing 40 minutes after midnight, and it has actually lost 30 minutes when it is showing 1am. So, it loses 20 minutes for every hour of REAL time and 30 minutes for each hour that passes on the misfortune clock.
When the misfortune clock is showing 4am, it has lost 120 minutes or 2 hours of time. So, when it shows 4am, it is actually 6am. Since that was 4 hours ago, it is now 10 am.
10 AM.
Each hour it displays corresponds to 2/3 hour. It ran for 4 / (2/3) = 6 hours. 6 + 4 = 10 AM.
It is 10:00 am. If it ran for 6 hours without stopping, it would have lost 6x20 minutes or 2 hours. Hence it would show 4 am. As it stopped 4 hours ago it is now 10 am.
Awesome puzzle. Took me time to figure it out.
I am so happy to read this. This is the kind of manual that needs to be given and not the random misinformation that's at the other blogs.
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10 am is wright answer !
10 am is the wright answer :)
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