A batch of saplings were transplanted in Zhanqi Cultural Square. If 21 plants are planted in each row, 20 rows should be planted; if 28 plants are planted in each row, how many rows should be planted.
战旗文化广场移栽一批树苗．如果每行栽21株，要栽20行，如果每行栽28株，要栽多少行．
<gadget id="calculator">20 / 28</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">21 * (5/7)</gadget>
<output>15</output>

<result>15</result>