One job can be done by A alone in 20 hours, and by B alone in 12 hours. How many hours will it take for A and B to do this job together?
一件工作，甲单独做20小时完成，乙单独做12小时完成，问甲乙合做，需几小时完成这件工作？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/20) + (1/12)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">1 / (2/15)</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>