A book, (1/5) of this book was read in the first 4 days, according to this calculation, how many days will it take to finish reading this book.
一本书，前4天看了这本书的(1/5)，照这样计算，看完这本书还要多少天．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/5) / 4</gadget>
<output>1/20 = around 0.05</output>

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

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<result>16</result>