You ride your bike to a beach that is 15 miles away. Your average speed on the way home is 5 miles per hour less than your average speed on the way to the beach. Write and equation that gives the total travel time t (in hours) as a function of your average speed r (in miles per hour) on the way to the beach. Then find the total travel time if you biked to the beach at an average speed of 15 miles per hour.