Xiaohua goes from place A to place B, walks, and travels 4.8 kilometers per hour; when returning, takes a car, and travels 72 kilometers per hour. How many kilometers per hour is Xiaohua's average round-trip speed?
小华从甲地去乙地，去时步行，每小时行4.8千米；回来时乘车，每小时行72千米．小华往返的平均速度是每小时多少千米？
<result>9</result>