It takes 3.5 seconds for a train to pass a road sign, 20 seconds to pass a 300-meter-long bridge, how many seconds does it take to pass through a 800-meter-long cave.
一列火车经过一个路标用3.5秒，通过一座长300米的桥用了20秒，它穿过长800米的山洞要多少秒．
<gadget id="calculator">20 - 3.5</gadget>
<output>16.5</output>

<gadget id="calculator">300 / 16.5</gadget>
<output>18.181818</output>

<gadget id="calculator">800 / 18.181818</gadget>
<output>44</output>

<gadget id="calculator">44 + 3.5</gadget>
<output>47.5</output>

<result>47.5</result>