It takes 15 days for A to complete a project alone, and 20 days for B to complete it alone. If A and B cooperate, how many days will it take to complete?
一项工程，甲单独做需要15天完成，乙单独做需要20天完成，如果由甲乙合作需要几天才可以完成？
<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>