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Source : GMATPrep At a certain instant in time, the number

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Joined: 30 Jun 2008
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Source : GMATPrep At a certain instant in time, the number [#permalink]  11 Oct 2008, 08:54
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100% (05:11) correct 0% (00:00) wrong based on 1 sessions
Source : GMATPrep

At a certain instant in time, the number of cars, N , traveling on a portion of a certain highway can be estimated by the formula

N=( 20*L*d)/(600+s^2)

where L is the number of lanes in the same direction, d is the length of the portion of the highway, in feet, and s is the average speed of the cars, in miles per hour. Based on the formula, what is the estimated number of cars traveling on a 21 mile portion of the highway if the highway has 2 lanes in the same direction and the average speed of the cars is 40 miles per hour? (5,280 feet = 1 mile)
(A) 155
(B) 96
(C) 80
(D) 48
(E) 24
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Re: PS: Estimated number of Cars [#permalink]  11 Oct 2008, 09:48
could you copy & paste the screenshot ?

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Re: PS: Estimated number of Cars [#permalink]  11 Oct 2008, 09:58
Not able to solve, can anybdy help me out ?
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Re: PS: Estimated number of Cars [#permalink]  11 Oct 2008, 10:13
I am not sure if I posted the question right. No screen-shot available

I googled to find this http://www.manhattangmat.com/forums/at- ... t4743.html

but not much help there I guess
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Re: PS: Estimated number of Cars [#permalink]  11 Oct 2008, 16:25
amitdgr wrote:
Source : GMATPrep

At a certain instant in time, the number of cars, N , traveling on a portion of a certain highway can be estimated by the formula

N=( 20*L*d)/(600+s^2)

where L is the number of lanes in the same direction, d is the length of the portion of the highway, in feet, and s is the average speed of the cars, in miles per hour. Based on the formula, what is the estimated number of cars traveling on a 21 mile portion of the highway if the highway has 2 lanes in the same direction and the average speed of the cars is 40 miles per hour? (5,280 feet = 1 mile)
(A) 155
(B) 96
(C) 80
(D) 48
(E) 24

N = (20*L*d)/(600+s^2)
N = (20*2*21x5280)/(600+40^2)
N = (20*2*21x5280)/2200
N = (2*21x48)
N = 96*21

Answer might be B but all answer choices are wrong because N is 96x21. The GmatPrep might have forgotten to include 21 in neumerator and simply put 5280.

I think I saw this question earlier but do not remember whether it was solved.
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Re: Source : GMATPrep At a certain instant in time, the number [#permalink]  18 Nov 2011, 11:32
First convert the feet to mile. So 1 mile = 5280 feet.
1/2 mile = 2640 feet
So substituting the values in given formula
(10 * 2 * 2640)/(1600 + 600)

= 48

ans: D
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Re: Source : GMATPrep At a certain instant in time, the number [#permalink]  18 Nov 2011, 12:58
This is a question from OG12 (#68) and is pretty straightforward. You have not copied it correctly which is why it is stumping everyone.

The correct question states "At a certain instant in time, the number of cars, N,
traveling on a portion of a certain highway can be
estimated by the formula (as given)

where L is the number of lanes in the same direction,
d is the length of the portion of the highway, in feet,
and s is the average speed of the cars, in miles per
hour. Based on the formula, what is the estimated
number of cars traveling on a 1/2-mile portion of the
highway if the highway has 2 lanes in the same
direction and the average speed of the cars is 40
miles per hour? (5,280 feet = 1 mile)"

So the portion is 1/2 mile NOT 20 miles. And thus the correct answer is D, after just plugging in numbers and converting 1/2 mile to feet.
Re: Source : GMATPrep At a certain instant in time, the number   [#permalink] 18 Nov 2011, 12:58
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