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
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.