A ship sails from Port A to Port B, travels 36km per hour along the current, and arrives in 15 hours. It returns to Port A along the original road, and travels 30km per hour on average against the current. How long does it take to return to Port A?
一艘轮船从甲港开往乙港，顺水航行每小时行36km，15小时到达，沿原路从乙港返回甲港，逆水航行平均每小时行30km，多长时间能够返回甲港？
<gadget id="calculator">15 / 30</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">36 * (1/2)</gadget>
<output>18</output>

<result>18</result>