The library originally had 1,500 sets of books, and after lending out x sets, another 200 sets were bought in, and the library now has 900 sets of books.
图书馆原有图书1500套，借出x套后，又进了200套，图书馆这时有图书900套．
<gadget id="calculator">200 - 900</gadget>
<output>-700</output>

<gadget id="calculator">1_500 + (-700)</gadget>
<output>800</output>

<result>800</result>