Xiao Ming reads a book. He reads 25% of the whole book on the first day and 20% of the whole book on the second day. He reads 20 more pages on the first day than on the second day. How many pages does this book have?
小明看一本书，第一天看了全书的25%，第二天看了全书的20%，第一天比第二天多看20页，这本书一共有多少页？
<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">20 / (1/20)</gadget>
<output>400</output>

<result>400</result>