15. Bald Qiang rides an electric bike from home to the hospital for an injection. On the way, there are only slopes, no flat roads. After Bald Qiang gets the injection, he returns the same way. The round trip takes a total of 5 hours. If he travels at 8 kilometers per hour uphill and 12 kilometers per hour downhill, then the distance between Bald Qiang's home and the hospital is $\qquad$ kilometers.