The cost of the trip is an increasing function, so I assume that you mean that the cost must not increase beyond 30 $/hr.
I solved the following equation, which has one real solution:
x is the speed in km/hr, so that cost is 30 $/hr. Any speed less than that would result in lower $/hr as well as lower cost of the trip.
The answer in my textbook says it's 97.9km/h. Are you sure that's right?