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An alarm of a certain clock rings every 20 minutes. If the clock's ala

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An alarm of a certain clock rings every 20 minutes. If the clock's ala  [#permalink]

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New post Updated on: 18 Aug 2017, 19:54
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An alarm of a certain clock rings every 20 minutes. If the clock's alarm first rings at 15:20, when will the 12th alarm of the clock ring?

A. 17:30
B. 18:00
C. 18:30
D. 19:00
E. 19:30

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Originally posted by MathRevolution on 16 Aug 2017, 02:43.
Last edited by MathRevolution on 18 Aug 2017, 19:54, edited 2 times in total.
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Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala  [#permalink]

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New post 16 Aug 2017, 02:46
MathRevolution wrote:
An alarm of a certain clock rings every 20 minutes. If the clock's alarm first rings at 15:20, when will the 12th alarm of the clock ring?

A. 17:30
B. 18:00
C. 18:30
D. 19:00
E. 19:30


Similar question: https://gmatclub.com/forum/an-alarm-of- ... 44589.html
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Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala  [#permalink]

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New post 16 Aug 2017, 03:57
2
the alarm rings every 20 mins. => d= 20 min= \(\frac{1}{3}\) hr
1st ring at 15:20. => a= 15:20= (15+\(\frac{1}{3}\)) hr=\(\frac{46}{3}\) hr
12th ring= t12= a+(12-1)d= a+11d= \(\frac{46}{3}\)+\(\frac{1}{3}\)= \(\frac{57}{3}\) hr= 19 hr= 19:00

Ans- D
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Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala  [#permalink]

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New post 16 Aug 2017, 04:07
@Buenel

My Answer is D here
just to prove D is correct and official answer is may be wrong.
first ring - 15:20
2nd 15:40
3rd 16:00
4th. 16:20
5th. 16:40
6. 17:00
7. 17:20
8. 17:40
9. 18:00
10. 18:20
11. 18:40
12th ring 19:00
Kudos for correct answer.
Also would like to mention i correctly answered https://gmatclub.com/forum/an-alarm-of- ... 44589.html question as well.
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Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala  [#permalink]

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New post 16 Aug 2017, 15:23
MathRevolution wrote:
An alarm of a certain clock rings every 20 minutes. If the clock's alarm first rings at 15:20, when will the 12th alarm of the clock ring?

A. 17:30
B. 18:00
C. 18:30
D. 19:00
E. 19:30

MathRevolution , or Bunuel , or some moderator, would you please check the answer choices? I got D, and so did others who posted here.

I ran it twice more, using different methods. Still got Answer D.
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Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala  [#permalink]

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New post 16 Aug 2017, 16:05
MathRevolution wrote:
An alarm of a certain clock rings every 20 minutes. If the clock's alarm first rings at 15:20, when will the 12th alarm of the clock ring?

A. 17:30
B. 18:00
C. 18:30
D. 19:00
E. 19:30



the 12th ring = 15:20 + 1/3 * 11 = 15:20 + 3:40 = 19:00

Answer: D
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Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala  [#permalink]

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New post 18 Aug 2017, 02:44
First alarm rings at 1520
Thus 12 alarm will ring in 11*20 =220 min
Thus 12th tym alarm will ring at 1520 + 3 hrs 40 min
Hence ans 1900 hrs
Ans D


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Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala  [#permalink]

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New post 18 Aug 2017, 03:36
MathRevolution wrote:
An alarm of a certain clock rings every 20 minutes. If the clock's alarm first rings at 15:20, when will the 12th alarm of the clock ring?

A. 17:30
B. 18:00
C. 18:30
D. 19:00
E. 19:30



Hi..
If a clock rings at interval of 20 minutes and starts at 1520...
Ans has to be at _ _ 20, _ _ 40 or _ _ 60, can't be at _ _ 30

So answer given is clearly wrong ..

Now ans is 11*20+ 1520h..
11*20/60= 11/3= 3+2/3.. so 3 hours and 60*2/3=40 min.
So 15:20 + 3 hour and 30 mins=1900

D

Editing the OA
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Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala  [#permalink]

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New post 20 Aug 2017, 18:25
=>The third alarm rings at 16:00, the sixth alarm rings at 17:00. Thus every third alarm rings every hour on the hour. The 12th alarm rings at 19:00.

Ans: D
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Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala &nbs [#permalink] 20 Aug 2017, 18:25
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