A book has been read (3/7), the number of pages that have been read is the number of unread pages.
一本书，已读了(3/7)，已读的页数是未读的多少．
<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">1 - (3/7)</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">(3/7) / (4/7)</gadget>
<output>3/4 = around 0.75</output>

<result>3/4 = around 0.75</result>