Problem Solving

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

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

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 + 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)

(A) 155
(B) 96
(C) 80
(D) 48
(E) 2

We are given the following:

N = the number of cars in a certain instant in time

L = number of lanes in the same direction

d = length of the portion of the highway, in feet

s = average speed of the cars, in miles per hour

We are given the following values for the variables:

d = ½ mile

L = 2 lanes

s = 40 mph

Before plugging these values into the equation, we must convert ½ mile to feet. Since we know that (5,280 feet = 1 mile), we know that:

½ mile = ½ x 5,280 = 2,640 feet

So now we can plug all this information into the equation to determine N, the estimated number of cars.

N = (20Ld)/(600 + s^2)

N = (20 x 2 x 2,640)/(600 + 40^2)

N = (40 x 2,640)/2,200

N = (4 x 264)/22

N = (2 x 264)/11

N = 528/11 = 48

Answer D