There are two layers of books on the bookshelf. There are 180 books on the upper layer, which is three times the number of books on the lower layer. How many books are there on the lower layer?
书架上有上下两层书，上层有180本书，是下层书本数的3倍，下层有多少本书？
<gadget id="calculator">180 / 3</gadget>
<output>60</output>

<result>60</result>