A courier rides an electric scooter that can move at 15 km/h only along an east–west road. The destination is a building located 3 km north and 8 km east of the courier’s current position. After riding x kilometers east on the road, the courier abandons the scooter and walks straight to the building at 5 km/h. Which value of x (to the nearest tenth of a kilometer) minimizes the courier’s travel time?
a) 9.1 km
b) 7.8 km
c) 6.9 km
d) 8.0 km