For a book, Haohao read 30% of the whole book on the first day and 50% of the whole book on the second day. It is known that he read 20 pages less on the first day than on the second day. How many pages is this book?
一本书，浩浩第一天读全书的30%，第二天读全书的50%，已知第一天比第二天少读20页，这本书多少页？
<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(1/2) - (3/10)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">20 / (1/5)</gadget>
<output>100</output>

<result>100</result>