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In this why are we not converting speed to feet?

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shibsriz@gmail.com Master | Next Rank: 500 Posts Default Avatar
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In this why are we not converting speed to feet?

Post Sun Jul 14, 2013 10:42 pm
70. 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= 20Ld
600+ s2
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 aA-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 = 1mile)

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ganeshrkamath Master | Next Rank: 500 Posts
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Post Tue Aug 20, 2013 8:16 pm
shibsriz@gmail.com wrote:
70. 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= 20Ld
600+ s2
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 aA-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 = 1mile)
This is a question from OG12 and is pretty straightforward. You have not copied it correctly.

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
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 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)
"

Solution:
N=(20*L*d)/(600+s^2)
L = 2
d = 1/2 miles = 1/2 * 5280 feet
s = 40 miles/hour

N = 20*2*1/2*5280/(600 + 40^2)
= 20*2*1/2*5280/(600 + 1600)
= 20*5280/2200
= 5280/110
= 528/11
= 48

The question explicitly says that d is in feet and s is in miles per hour.

Cheers

_________________
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Kelley School of Business (Class of 2016)
GMAT Score: 750 V40 Q51 AWA 5 IR 8
https://www.beatthegmat.com/first-attempt-750-in-2-months-t268332.html#688494

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GMAT/MBA Expert

Post Tue Aug 20, 2013 4:14 pm
Hi ProGMAT,

We don't convert speed (from miles/hr to feet/hr) because the question doesn't ask us to. Notice the wording:

L is the number of lanes
d is the distance IN FEET
s is the speed IN MILES PER HOUR

Just follow the instructions, plug in the numbers and you'll have the answer to the question.

GMAT assassins aren't born, they're made,
Rich

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Contact Rich at Rich.C@empowergmat.com

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GMAT/MBA Expert

Post Sun Jul 14, 2013 10:56 pm
Hi shibsriz,

While this question is certainly wordy, you're given a formula, told what the variables are and given the numbers to plug in...

L = number of lanes = 2
d = length of highway in feet = 1 mile = 5280 feet
s = average speed of cars = 40

Now, plug these values into the formula and solve.

GMAT assassins aren't born, they're made,
Rich

_________________
Contact Rich at Rich.C@empowergmat.com

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ProGMAT Senior | Next Rank: 100 Posts
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Target GMAT Score:
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Post Tue Aug 20, 2013 9:36 am
Rich.C@EMPOWERgmat.com wrote:
plug these values into the formula and solve.

Rich
In this question I am confused that why are we not converting speed to feet(its in mile only)?

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