500 people from Qingquan Primary School participate in the entrance ceremony of the sports meeting. There are 20 people in a row, the distance between the two rows is 3 meters, and the rostrum is 18 meters. How long does it take for them to pass the rostrum at a speed of 30 meters per minute?
清泉小学500人参加运动会入场式，每20人一行，两行之间距离3米，主席台18米，他们以每分钟30米的速度通过主席台，需要多少分钟．
<gadget id="calculator">500 / 20</gadget>
<output>25</output>

<gadget id="calculator">25 - 1</gadget>
<output>24</output>

<gadget id="calculator">24 * 3</gadget>
<output>72</output>

<gadget id="calculator">72 + 18</gadget>
<output>90</output>

<gadget id="calculator">90 / 30</gadget>
<output>3</output>

<result>3</result>