Engineers rushed to repair a section of railway, and it took 12 hours for the first company to repair it alone, and it took 15 hours for the second company to repair it alone. Due to the urgency of the task, there are now two joint repairs. How many hours can the task be completed?
工程兵抢修一段铁路，一连单独修要12小时才能完成，二连单独修要15小时才能完成．因任务紧急，现由两个连合修，多少小时可以完成任务？
<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">1 / (3/20)</gadget>
<output>20/3 = around 6.666667</output>

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