Xiaopang reads a book and has already read 39 pages, and the remaining pages are exactly 20 times the number of pages he has read. How many pages are there in this book?
小胖看一本书，已经看了39页，剩下的页数正好是已经看的20倍，这本书共有多少页？
<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<gadget id="calculator">39 * 21</gadget>
<output>819</output>

<result>819</result>