Tommy and Jena are taking a 60 mile bike ride in the mountains together. Halfway on the route, Jena crashes her bike. To go back to the start of the trail, they decide to share Tommy's bike. Tommy will ride the bike first, park it, then finish the route by running. When Jena reaches the bike, she will get on and start riding the rest of the route. They can both ride at 15 mph and run at 6 mph. If Tommy and Jena return to the trailhead at the same time, for what portion of the time spent traveling back is the bike being used?