The bus departs every 20 minutes. Uncle Wang wanted to take the 8:30 bus, but when he arrived at the station, it was already 8:37. How many more minutes would he have to wait before he could get on the next bus?
公交车每20分钟开出一班，王大爷想乘坐8：30那一班，可是他到车站时，已经是8：37了，他要再等多少分钟才能搭上下一班车．
<gadget id="calculator">37 - 30</gadget>
<output>7</output>

<gadget id="calculator">20 - 7</gadget>
<output>13</output>

<result>13</result>