A book has been read (2/5), and there are still 15 pages left unread. The number of pages viewed is the number of pages not viewed.
一本书，已经看了(2/5)，还有15页没看．已看页数是没看页数的多少．
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">(2/5) / (3/5)</gadget>
<output>2/3 = around 0.666667</output>

<result>2/3 = around 0.666667</result>