Larry's construction team is working on a project to build several small buildings. They want the overall average time spent on each building to be 15 hours or less. So far, they have spent the following amounts of time on the first four buildings: 17 hours, 10 hours, 19 hours, and 13 hours. How many hours should they spend building the fifth building in order to achieve their goal of an average of 15 hours or less?