Drink a glass of water (1/20), then add 20 ml to fill up, if drink (2/5), how many ml to add to fill up?
一杯水喝掉(1/20)，再加入20毫升就注满，若喝掉(2/5)，再加入多少毫升就注满？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">20 / (1/20)</gadget>
<output>400</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">400 * (2/5)</gadget>
<output>160</output>

<result>160</result>