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At exactly what time past 7:00 will the minute and hour

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Re: At exactly what time past 7:00 will the minute and hour [#permalink]

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New post 19 Sep 2017, 04:05
sinhap07 wrote:
Engr2012 wrote:
baharemustafa wrote:
At exactly what time past 7:00 will the minute and hour hands of an accurate working clock be precisely perpendicular to each other for the first time?

(A) 20 13/21 minutes past 7:00
(B) 20 13/17 minutes past 7:00
(C) 21 3/23 minutes past 7:00
(D) 21 9/11 minutes past 7:00
(E) 22 4/9 minutes past 7:00


5.5 is the angle between minute n hour, this is what I was taught...so shouldn't it be solve by dividing 90 with 5.5?


That would have been the case if your initial difference between the hour and the minute hand was = 0 degrees or in other words, both minute and hour hands were at the same location. But as per the question, you are asked for time AFTER 7:00. At 7:00, the angle between the hour and the minute hand is 210 degrees. you need to take this into account as well.

So in order for the difference to decrease to 90 degrees, the minute hand must eat away this difference of 210-90 = 120 degree at the rate of 5.5 degrees per minute ---> 120/5.5 = 21 9/11 minutes.

Thus, D is the correct answer.

Hope this helps.

P.S.: If what you are saying was correct, then you should have calculated 16 4/11 minutes and this is not present in the given options. This should have told you that you are making a mistake somewhere.



Hi
Why shouldnt I consider the initial distance between the hour and minute hand be 150 degrees?


The hands of the clock move so that 150 degree angle increases and 210 degree angle decreases, so that will be the angle which will be 90 degree.
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Re: At exactly what time past 7:00 will the minute and hour [#permalink]

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New post 16 Oct 2017, 00:51
Bunuel karishma - Can we look at this question from an angle of relative change as at 7.00 PM the change in the angle would be 210 deg and to make it perpendicular post 7 PM as per the question stem the total relative change would be 300 (210+90).

Which can be derived as 300/360*11/2 which is the total change in the hourly and the minute hand of the clock.

If yes, i have solved the question as 300/360*11/2 which is coming at 55/12 and i am stuck here!

Thanks in advance.

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Re: At exactly what time past 7:00 will the minute and hour [#permalink]

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siddyj94 wrote:
Bunuel karishma - Can we look at this question from an angle of relative change as at 7.00 PM the change in the angle would be 210 deg and to make it perpendicular post 7 PM as per the question stem the total relative change would be 300 (210+90).

Which can be derived as 300/360*11/2 which is the total change in the hourly and the minute hand of the clock.

If yes, i have solved the question as 300/360*11/2 which is coming at 55/12 and i am stuck here!

Thanks in advance.


Yes, you can. I have given the link to the relevant post on the previous page.
Here is what it says: the minute hand covers 11/2 degrees per minute relative to the hour hand.

Note that position of the hour hand and the minute hand at 7. The larger angle between them is 210 degrees which needs to be brought down to 90 degrees (Think why - the hour hand will barely move but the minute hand will need to cover most of the difference between them and when it is closer to the hour hand, at some time, angle between them will be 90 degrees)

Relative to the hour hand, the minute hand has to cover 210 - 90 = 120 degrees.

Time taken for that will be \(\frac{120}{(11/2)} = \frac{240}{11} = 21(\frac{9}{11})\) mins

Answer (D)
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Re: At exactly what time past 7:00 will the minute and hour [#permalink]

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New post 06 Dec 2017, 13:05
Hi All,

When dealing with a clock face, there are several pieces of information to consider:
1) A clock face is 360 degrees, so each "hour mark" represents 30 degrees and each "minute mark" represents 6 degrees.
2) As the minute hand moves, the hour hand ALSO moves (a little). Over the course of 1 full hour of time, the minute hand will move 360 degrees and the hour hand will move 30 degrees.
3) For each MINUTE that the minute hand moves, the hour hand moves 0.5 DEGREES.

To find the first time AFTER 7:00 that the hour hand and minute hand are PERPENDICULAR (meaning that their positions are 90 degrees apart), you might find it useful to work in small increments. From the answer choices, we see that the correct answer has to be fairly close to 7:20, so we'll 'work up' to there:

At 7:00...
the minute hand is at 0 degrees
the hour hand is at 210 degrees

At 7:20...
the minute hand is at 120 degrees
the hour hand has moved to 210 + 10 = 220 degrees
So the arms are getting closer to where we want them to be, but they're not there yet (they're 100 degrees apart)...

At 7:21...
the minute hand is at 126 degrees
the hour hand has moved to 220 + 0.5 = 220.5 degrees
So the arms are getting closer to where we want them to be, but they're not there yet (now they're 94.5 degrees apart)...

At 7:22...
the minute hand is at 132 degrees
the hour hand has moved to 220.5 + 0.5 = 221 degrees
Here, the minute hand has 'overshot' just a little bit (now they're 89 degrees apart)... Thus, the correct answer has to be just a little 'earlier' than 7:22

Final Answer:
[Reveal] Spoiler:
D


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Re: At exactly what time past 7:00 will the minute and hour   [#permalink] 06 Dec 2017, 13:05

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