After checking that the gas tank of a car is filled to its capacity, a person proves the following:

Travelling d miles within a city, the number of gallons of gas left in the tank can be given by the function g(d)=15-d/20.

He can cover 3 more miles per gallon of gas on a highway within the city.

What is the maximum distance the car can travel on the highway with the tank filled to its capacity?