To make 120 parts, the master alone takes 15 hours to complete, and the apprentice alone takes 20 hours to complete. If the master and apprentice cooperate, how many hours can it be completed?
做120个零件，师傅单独做要15小时完成，徒弟单独做要20小时完成，如果师徒二人合作，几小时可以完成？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/15) + (1/20)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">1 / (7/60)</gadget>
<output>60/7 = around 8.571429</output>

<result>60/7 = around 8.571429</result>