A piece of iron pipe is 20 meters long, and it takes 5 minutes to saw every 4 meters.
一根铁管长20米，每隔4米锯开一段，每锯一次要用5分钟，全部锯完要用多少分钟．
<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<result>20</result>