A ship travels 15 kilometers per hour, and it travels 72 kilometers upstream in 6 hours. How many hours does it take if it travels the same distance along the current?
一艘轮船每小时行15千米，它逆水6小时行了72千米，如果它顺水行驶同样长的航程需要几个小时？
<gadget id="calculator">72 / 6</gadget>
<output>12</output>

<gadget id="calculator">15 - 12</gadget>
<output>3</output>

<gadget id="calculator">3 + 15</gadget>
<output>18</output>

<gadget id="calculator">72 / 18</gadget>
<output>4</output>

<result>4</result>