There is a willow tree planted every 9 meters on one side of the road. Zhang Jun saw a total of 501 trees in 5 minutes by car. How many kilometers did the car travel per hour?
马路的一边每相隔9米栽有一棵柳树，张军乘汽车5分钟共看到501棵树，问汽车每小时行多少千米？
<result>21.6</result>