The height of 15 dictionaries is 420 mm. If the height of a stack of dictionaries is 504 mm, how many dictionaries are there?
15本字典摆在一起高420毫米，若有一摞字典的高度是504毫米，问有多少本字典？
<gadget id="calculator">420 / 15</gadget>
<output>28</output>

<gadget id="calculator">504 / 28</gadget>
<output>18</output>

<result>18</result>