A cistern has two water inlets, A and B. If only the water inlet A is opened, the cistern can be filled with water in 20 hours; only the water inlet B is opened, and the cistern can be filled with water in 30 hours. If the water inlets A and B are opened at the same time, how long will it take to fill the reservoir with water?
某蓄水池有A，B两个进水口，仅打开进水口A，用20小时可以把蓄水池注满水；仅打开进水口B，用30小时可以把蓄水池注满水．如果同时打开A，B两进水口，需多长时间可以把蓄水池注满水？
<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>