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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
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    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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    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.

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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)?

    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.

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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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