The fuel carried by a steamboat can take up to 9 hours. It travels 15 kilometers per hour downstream and 12 kilometers per hour upstream. This steamboat needs to go back as far as the maximum kilometers it can travel.
一只汽船所带的燃料，最多用9小时，去时顺流每小时行15千米，回来是逆流每小时行12千米，这只汽船最多行出多少千米就需往回开．
<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">9 * (4/9)</gadget>
<output>4</output>

<gadget id="calculator">15 * 4</gadget>
<output>60</output>

<result>60</result>