Xiao Ming reads a book and has already read 80 pages, which happens to be 4/5 of the whole book. How many pages are left unread?
小明读一本书，已经读了80页，恰好是全书的(4/5)，还剩下多少页没有读？
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">80 / (4/5)</gadget>
<output>100</output>

<gadget id="calculator">100 - 80</gadget>
<output>20</output>

<result>20</result>