The fuel carried by a steamboat can take up to 6 hours. It travels 15 kilometers per hour downstream, and travels 12 kilometers per hour upstream when it returns.
一只汽船所带的燃料，最多用6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">5 + 4</gadget>
<output>9</output>

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

<gadget id="calculator">15 * 6 * (4/9)</gadget>
<output>40</output>

<result>40</result>