There are 200 books in the library, which are placed on three floors. The first floor has 10 books less than the second floor, and the second floor has 30 books less than the third floor. How many books are there on the first floor?
图书室有书200本，分三层放置．第一层比第二层少10本，第二层比第三层少30本，第一层放了多少本？
<gadget id="calculator">200 - 10 - 10 - 30</gadget>
<output>150</output>

<gadget id="calculator">150 / 3</gadget>
<output>50</output>

<result>50</result>