For a manuscript, it takes 12 hours for A to type alone, and 15 hours for B to type alone. A and B worked together for 2 hours, how much of this manuscript was completed.
一份稿件，甲单独打需要12小时，乙单独打需要15小时．甲、乙合作了2小时，共完成了这份稿件的多少．
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

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

<gadget id="calculator">(3/20) * 2</gadget>
<output>3/10 = around 0.3</output>

<result>3/10 = around 0.3</result>