A batch of garments can be completed in 30 days by A alone, and 20 days by B alone. How many days will it take for A and B to work together?
一批服装由甲单独做需30天才能完成，由乙单独做需20天完成．甲、乙合干需要多少天能完成？
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

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

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

<result>12</result>