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

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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 5866
GMAT 1: 760 Q51 V42
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An alarm of a certain clock rings every 20 minutes. If the clock's ala  [#permalink]

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Updated on: 18 Aug 2017, 19:54
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35% (medium)

Question Stats:

60% (00:52) correct 40% (00:49) wrong based on 113 sessions

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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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"Only $99 for 3 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" 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. Math Expert Joined: 02 Sep 2009 Posts: 47168 Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala [#permalink] ### Show Tags 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 _________________ Intern Joined: 22 Apr 2015 Posts: 28 WE: Business Development (Internet and New Media) Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala [#permalink] ### Show Tags 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 Senior Manager Joined: 29 Jun 2017 Posts: 497 GPA: 4 WE: Engineering (Transportation) Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala [#permalink] ### Show Tags 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. _________________ Give Kudos for correct answer and/or if you like the solution. SC Moderator Joined: 22 May 2016 Posts: 1835 Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala [#permalink] ### Show Tags 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. _________________ In the depths of winter, I finally learned that within me there lay an invincible summer. -- Albert Camus, "Return to Tipasa" SVP Joined: 26 Mar 2013 Posts: 1730 Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala [#permalink] ### Show Tags 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 Intern Joined: 18 Aug 2017 Posts: 2 Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala [#permalink] ### Show Tags 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 Sent from my iPhone using GMAT Club Forum mobile app Math Expert Joined: 02 Aug 2009 Posts: 6278 Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala [#permalink] ### Show Tags 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 _________________ 1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372 2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html 3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html GMAT online Tutor Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 5866 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: An alarm of a certain clock rings every 20 minutes. If the clock's ala [#permalink] ### Show Tags 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 _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$99 for 3 month Online Course"
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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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