ship sails from A to B, travels 20 kilometers per hour, arrives in 15 hours, turns around immediately after arriving at B, and returns to A in only 12 hours. Find the average round trip speed of the ship.
一艘轮船从甲地开往乙地，每小时行使20千米，15小时后到达，到达乙地后立即转向返航，只用了12小时就回到甲地．求这艘轮船往返的平均速度．
<gadget id="calculator">15 + 12</gadget>
<output>27</output>

<gadget id="calculator">2 / 27</gadget>
<output>2/27 = around 0.074074</output>

<gadget id="calculator">20 * 15 * (2/27)</gadget>
<output>200/9 = around 22.222222</output>

<result>200/9 = around 22.222222</result>