There are three students who need the teacher to correct their homework. The time for each teacher to help is as follows: Lingling needs 5 minutes, Lan Lan needs 3 minutes, and Xing Xing needs 15 minutes. To minimize the total waiting time of the three students, how many minutes are needed at least.
有三位学生要找老师批改作业，每个需要老师帮助的时间如下：玲玲要5分钟，兰兰要3分钟，星星15分钟，要使三位同学等候的时间总和最少，最少需要多少分钟．
<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">9 + 10 + 15</gadget>
<output>34</output>

<result>34</result>