The school library originally had 907 science and technology books, 273 were lent out, and 209 were returned. How many science and technology books are there in the library now?
学校图书馆，原有科技书907本，借出273本，还回209本，现在图书馆里有科技书多少本？
<gadget id="calculator">907 - 273</gadget>
<output>634</output>

<gadget id="calculator">634 + 209</gadget>
<output>843</output>

<result>843</result>