When Xiao Ming reads a book, he has already read 105 pages, and the remaining pages are 15 pages a day, and he needs 20 days to finish. How many pages are there in this book?
小明看一本书，已经看了105页，剩下的每天看15页，还要看20天看完．这本书一共多少页？
<gadget id="calculator">15 * 20</gadget>
<output>300</output>

<gadget id="calculator">300 + 105</gadget>
<output>405</output>

<result>405</result>