You say, at one point "range" and, at another, "domain". I presume you are thinking of the number of passengers per day as a function of the fare so I would say that, since they have "space to serve up to 15,000 passengers per day" the **range** will be 0 to 15,000. At the same time, you are told that, at $20 fare, they serve 10,000 passengers per day and "if the fare increases by $0.50 200 fewer people will ride the bus". (This problem does not say "per day" but I assume that is intended.) That is, taking "P" to be the number of passengers and "f" the fare, P= 10000- 200((f- 20)/0.50= 10000- 400(f- 20).

There cannot be less than 0 passengers so the fare cannot be more than f such that 10000- 400(f- 20)= 0. That is, 400(f- 20)= 10000, f- 20= 25 or f= $45.00. On the other hand, since there cannot be more than 15000 passengers, the fare cannot be les than f such that 10000- 400(f- 20)= 15000. 400(f- 20)= -5000, f- 20= -12.5, f= $7.50. The domain is $7.50 to $45.00.

But you really don't need to know either domain or range to answer this question as you appear to have discovered.