There are 21 poplar trees in the campus, which are pine trees (3/4), and pagoda trees are pine trees (4/7). How many locust trees are there on campus?
校园里有杨树21棵，是松树的(3/4)，槐树是松树的(4/7)．校园里有多少棵槐树？
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">21 / (3/4)</gadget>
<output>28</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">28 * (4/7)</gadget>
<output>16</output>

<result>16</result>