A project can be completed by A alone in 3 days (1/4), and B alone can complete it (1/5). Two people work together, how many days to complete the entire project?
一件工程，甲独做3天可以完成(1/4)，乙独做可以完成(1/5)．二人合作，几天完成全部工程？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/4) / 3</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/5) / 3</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>