Three customers came to the mobile phone maintenance department. According to the degree of damage to their mobile phones, the repairs took 20 minutes, 15 minutes, and 10 minutes respectively. What is the minimum sum of the waiting time of these three customers?
手机维修部来了三位顾客，根据他们手机的损坏程度修理分别需要20分钟、15分钟、10分钟，这三位顾客等候时间的总和最少是多少分钟．
<gadget id="calculator">10 * 3</gadget>
<output>30</output>

<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<gadget id="calculator">30 + 30 + 20</gadget>
<output>80</output>

<result>80</result>