Suppose that in a certain city, the number of people N using the mass transportation system is a function of the fare f and the daily cost of downtown parking p, so that N =N(f, p). Interpret each of the following.
(a) N(5, 20) = 65,000
(b) ∂N/∂f (5, 20) = - 4000
(c) ∂N/∂p (5, 20) = 2500