For a batch of parts, the master processes 8 hours a day and completes it in 15 days; the apprentice processes 9 hours a day and completes it in 20 days. If two people work together and each work 6 hours a day, how many days will it take to complete?
一批零件，师傅每天加工8小时，15天完成；徒弟每天加工9小时，20天完成．如果二人合作，每人每天都加工6小时，需要几天完成？
<gadget id="calculator">8 * 15</gadget>
<output>120</output>

<gadget id="calculator">1 / 120</gadget>
<output>1/120 = around 0.008333</output>

<gadget id="calculator">9 * 20</gadget>
<output>180</output>

<gadget id="calculator">1 / 180</gadget>
<output>1/180 = around 0.005556</output>

<gadget id="calculator">(1/120) + (1/180)</gadget>
<output>1/72 = around 0.013889</output>

<gadget id="calculator">(1/72) * 6</gadget>
<output>1/12 = around 0.083333</output>

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

<result>12</result>