The fuel carried by a steamboat can take up to 6 hours. It travels 18 kilometers per hour downstream and 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18千米，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">1 / 18</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/18) + (1/12)</gadget>
<output>5/36 = around 0.138889</output>

<gadget id="calculator">6 / (5/36)</gadget>
<output>216/5 = around 43.2</output>

<result>216/5 = around 43.2</result>