A pen can be exchanged for 4 writing brushes, 2 writing brushes can be exchanged for 5 pencils, and how many pens can be exchanged for 20 pencils.
一支钢笔换4只毛笔，2只毛笔能换5只铅笔，20只铅笔能换多少只钢笔．
<gadget id="calculator">20 / 5</gadget>
<output>4</output>

<gadget id="calculator">2 / 4</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">4 * (1/2)</gadget>
<output>2</output>

<result>2</result>