A movie theater has two exits, A and B. It takes 20 minutes for all the spectators to leave the arena if Exit A is opened alone; it takes 30 minutes for all spectators to leave the arena if Exit B is opened only. If both exits are open at the same time, how many minutes will it take for all spectators to leave?
某电影院有甲、乙两个出口．单开甲出口，全部观众离场需要20分钟；单开乙出口，全部观众离场需要30分钟．如果两个出口同时开放，多少分钟能让所有观众离场？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / (1/12)</gadget>
<output>12</output>

<result>12</result>