Xiaohua reads a book, and every day he reads 20% of the book and there are 40 pages more. In this way, he just finished reading it in 4 days. How many pages does the book have?
小华看一本书，每天看这本书的20%还多40页，这样4天刚好看完，这本书共有多少页？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">40 * 20</gadget>
<output>800</output>

<result>800</result>