Xiao Ming reads a book in three days. He reads 25% of the book on the first day and 30% of the book on the second day. He reads 21 more pages on the second day than on the first day. How many pages in this book are there?
小明三天看一本书，第一天看了全书的25%，第二天看了全书的30%，第二天比第一天多看了21页，这本书共多少页？
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(3/10) - (1/4)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">21 / (1/20)</gadget>
<output>420</output>

<result>420</result>