The library bought a batch of books, and after filling 4 bookshelves with these books, there are still 50 books left. If an average of 200 books are placed on each shelf, how many books are there in total?
图书室买回来一批图书，把这些图书放满4个书架后，还剩50本．如果平均每个书架放200本书，买来的这批图书一共有多少本？
<gadget id="calculator">200 * 4</gadget>
<output>800</output>

<gadget id="calculator">800 + 50</gadget>
<output>850</output>

<result>850</result>