There are 180 books on the upper shelf of a bookshelf, which is twice that of the lower shelf. How many books are there on the lower shelf? (solution by equation)
一个书架上层有180本书，是下层的2倍，书架下层有多少本书？(用方程解)
<gadget id="calculator">180 / 2</gadget>
<output>90</output>

<result>90</result>