A project can be completed by A alone in 20 days and by B in 15 days. If A and B work together, how many days can the whole project be completed?
一项工程，甲单独做20天完成，乙单独做15天完成．甲、乙合做，多少天可以完成全部工程？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

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

<gadget id="calculator">(1/20) + (1/15)</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>