A book has 200 pages, 20% of which have been read, and how many pages are left unread.
一本书200页，读了20%，还剩下多少页没有读．
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">200 * (4/5)</gadget>
<output>160</output>

<result>160</result>