Xiao Ming and his father go to get a haircut together. It is known that it takes 30 minutes for an adult to get a haircut, and 20 minutes for a child to get a haircut.
小明和爸爸一起去剪头发，已知成人剪发需要30分钟，儿童剪发需要20分钟，店里只有一位理发师，怎样安排可以使两人等待的时间总和最少？
<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">40 + 30</gadget>
<output>70</output>

<result>70</result>