It takes 15 hours for Xiao Ming to type a manuscript alone, and 12 hours for Xiao Gang. If the two of them work together, how many hours can they finish the manuscript?
打一份文稿，单独打小明要15小时，小刚要12小时，如果两人合打，几小时后可以完成这份文稿？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

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

<gadget id="calculator">(1/15) + (1/12)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">1 / (3/20)</gadget>
<output>20/3 = around 6.666667</output>

<result>20/3 = around 6.666667</result>