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The alarm of a certain clock rings every 15 minutes. If the

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by Max@Math Revolution » Fri May 25, 2018 1:31 am

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[GMAT math practice question]

The alarm of a certain clock rings every 15 minutes. If the alarm first rings at 12:15, when will it ring for the 24th time?

A. 17:30
B. 18:00
C. 18:30
D. 19:00
E. 19:30
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by GMATGuruNY » Fri May 25, 2018 2:54 am
Max@Math Revolution wrote:[GMAT math practice question]

The alarm of a certain clock rings every 15 minutes. If the alarm first rings at 12:15, when will it ring for the 24th time?

A. 17:30
B. 18:00
C. 18:30
D. 19:00
E. 19:30
After the first ring at 12:15, 23 additional rings are required to yield a total of 24 rings.
Since the clock rings every 1/4 hour, and 23 additional rings are required, the total time needed for 23 additional rings = (¼)(23) = 5 ¾ hours = 5 hours, 45 minutes.
Thus:
24th ring = (12:15) + (5:45) = 18:00.

The correct answer is B.
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by Max@Math Revolution » Sun May 27, 2018 5:05 pm
=>
It takes 1 hour = 4 x 15 minutes for every fourth alarm to ring. The time period starts at fifteen minutes before 12:15, or 12:00. So:
The 4th alarm rings at 13:00.
The 8th alarm rings at 14:00.
The 12th alarm rings at 15:00.
The 16th alarm rings at 16:00.
The 20th alarm rings at 17:00.
The 24th alarm rings at 18:00.

Therefore, the answer is B.
Answer: B
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by Scott@TargetTestPrep » Tue May 29, 2018 8:35 am
Max@Math Revolution wrote:
The alarm of a certain clock rings every 15 minutes. If the alarm first rings at 12:15, when will it ring for the 24th time?

A. 17:30
B. 18:00
C. 18:30
D. 19:00
E. 19:30
One hour has four 15-minute intervals. The clock rings 4 times every hour. Since it rings the 1st time at 12:15, the 5th time it will ring is at 13:15, the 9th time at 14:15, the 13th time at 15:15, the 17th time at 16:15, the 21st time at 17:15 and the 25th time at 18:15. So the 24th time must ring 15 minutes prior to 18:15, or at 18:00.

Answer: B

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by swerve » Tue May 29, 2018 10:49 am
The time can be found using the AP formula t_n = a + (n-1) * d

t_n = 12:15 + (24 - 1) * 15 = 12:15 + 23*15

Now 23*15 minutes is equivalent to (23*15)/60 = 5 hr 45 minutes.

Hence, the clock will ring at = 12:15 + 5:45 = 18:00. Option B.
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