The library buys back a batch of books, and after filling 4 bookshelves with these books, there are still 50 books left. If an average of 205 books are placed on each shelf, how many books are there in total?
图书室买回一批图书，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205本，买来的这批图书一共有多少本？
<gadget id="calculator">205 * 4</gadget>
<output>820</output>

<gadget id="calculator">820 + 50</gadget>
<output>870</output>

<result>870</result>