Derrick and Mark are brothers who live in the same house. Mark is at home, and Derrick is 5 miles away from home. Both boys begi
n riding their bikes at the same time. Derrick rides directly home at a constant rate of 15 mph, and Mark rides away from home at a constant rate of 18 mph. Let d represent distance from home, and t represent time in hours. Which system models this situation?
