There are 2 buses sharing the same stop sign. The first bus departs every 5 minutes, and the second bus departs every 4 minutes. If the two buses depart at the same time, at least how many minutes have passed before they depart at the same time.
有2路公共汽车共用一个站牌，一路车5分钟发一次车，二路车4分钟发一次车，如果两车同时出发，至少经过多少分钟后它们又同时发车．
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<result>20</result>