students line up to get food, if each student spends 3 minutes getting food, how long is the waiting time of the last student (from the start of queuing to the start of eating)?
5位同学排队打饭，若每位同学花了3分钟打饭，则最后一位同学等待的时间(从开始排队到开始吃饭)是多少分钟．
<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<result>15</result>