A small boat sails from port A to port B. It travels 18 kilometers per hour along the current and 12 kilometers per hour against the current when it returns. What is the average speed of the boat going back and forth in kilometers per hour?
一只小船从甲港开往乙港，去时顺水每小时行18千米，返回时逆水每小时行12千米，这只小船往返的平均速度是每小时多少千米
<result>14.4</result>