The school organizes students to have morning gymnastics training. If there are 15 people in each row, they need to stand in 28 rows. If there are 20 people in each row, how many rows do they need to stand in?
学校组织学生进行早体操训练，如果每列15人，需站28列，如果每列站20人，需站多少列？
<gadget id="calculator">28 * 15</gadget>
<output>420</output>

<gadget id="calculator">420 / 20</gadget>
<output>21</output>

<result>21</result>