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speed

by grandh01 » Tue Sep 04, 2012 8:23 pm
Due to construction, the speed limit
along an 8-mile section of highway is
reduced from 55 miles per hour to 35
miles per hour. Approximately how
many minutes more will it take to
travel along this section of highway at
the new speed limit than it would have
taken at the old speed limit ?
(A) 5
(B) 8
(C) 10
(D) 15
(E) 24

oa is B
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by everything's eventual » Tue Sep 04, 2012 8:44 pm
Should'nt the answer be 5?
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by avada » Tue Sep 04, 2012 9:17 pm
Its asking for an approximation. So I rounded the 55 to 60 and the 35 to 30.
So at 60 mph you are going 1 mile minute so it will take 8 minutes. At the new speed you are going 30 so it takes twice the time meaning 16. 16-8=8

i may be wrong though
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by vk_vinayak » Tue Sep 04, 2012 9:33 pm
grandh01 wrote:Due to construction, the speed limit
along an 8-mile section of highway is
reduced from 55 miles per hour to 35
miles per hour. Approximately how
many minutes more will it take to
travel along this section of highway at
the new speed limit than it would have
taken at the old speed limit ?
(A) 5
(B) 8
(C) 10
(D) 15
(E) 24

oa is B
Distance = Rate * Time => Time = Distance/Speed

Time taken to cover 8 miles with the speed of 55mph = 8/55 Hrs
Time taken to cover 8 miles with the speed of 35mph = 8/35 Hrs

Difference in time = (8/35) - (8/55) hrs = 32/385 hrs = 60 * (32/385) minutes = (approx) 5 minutes.

Ans A.
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by everything's eventual » Tue Sep 04, 2012 9:37 pm
@ avada : I don't think that we can apprx it this way.

60 is double of 30 but according to actual figures, 55 is nowhere close to double of 35. If we want to get apprx values then after the approximation, the ratio of new values should be close to 55 : 35.
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by Anurag@Gurome » Tue Sep 04, 2012 9:51 pm
grandh01 wrote:Due to construction, the speed limit
along an 8-mile section of highway is
reduced from 55 miles per hour to 35
miles per hour. Approximately how
many minutes more will it take to
travel along this section of highway at
the new speed limit than it would have
taken at the old speed limit ?
(A) 5
(B) 8
(C) 10
(D) 15
(E) 24

oa is B
When speed was 55 mph, time taken to cross 8 miles stretch = 8 * 60/55 = 8 * 12/11 = 96/11 = 8.72 mins
When speed was 55 mph, time taken to cross 8 miles stretch = 8 * 60/35 = 8 * 12/7 = 13.71

Difference in time = 13.71 - 8.72 = 5

The correct answer is A.
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by GMATGuruNY » Wed Sep 05, 2012 12:15 am
grandh01 wrote:Due to construction, the speed limit
along an 8-mile section of highway is
reduced from 55 miles per hour to 35
miles per hour. Approximately how
many minutes more will it take to
travel along this section of highway at
the new speed limit than it would have
taken at the old speed limit ?
(A) 5
(B) 8
(C) 10
(D) 15
(E) 24
Slower rate : faster rate = 35/55 = 7/11.
Rate and time are RECIPROCALS.
Since the slower rate is 7/11 of the faster rate, the time at the slower rate will be 11/7 of the time at the faster rate.
Thus, the time at the slower rate will be 4/7 LONGER than the time at the faster rate:
4/7 * 8/55 ≈ 1/2 * 1/6 ≈ 1/12.
Thus, the trip will take approximately 1/12 of an hour longer -- in other words, 5 more minutes -- at the slower rate.

The correct answer is A.
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