A bookshelf has 20 books on the first floor, 31 books on the second floor, and the same number on the third floor as the first floor. How many books are there in this bookshelf?
一个书架，第一层有20本书，第二层有31本书，第三层和第一层同样多，这个书架共有多少本书．
<gadget id="calculator">20 + 31 + 20</gadget>
<output>71</output>

<result>71</result>