project can be completed by A alone in 60 days and by B in 15 days. How many days does it take A and B to cooperate to complete (2/3) of the whole project?
一项工程，甲单独做60天完成，乙单独做15天完成．要完成全部工程的(2/3)需要甲、乙合作多少天．
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

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

<gadget id="calculator">(1/60) + (1/15)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(2/3) / (1/12)</gadget>
<output>8</output>

<result>8</result>