If A alone takes 15 days to complete a task, and B alone takes 12 days to complete it, how much can the two work together in 4 days?
一项工作，甲单独完成需要15天，乙单独完成需要12天，两人合作4天可以完成这项工作的多少．
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/15) + (1/12)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(3/20) * 4</gadget>
<output>3/5 = around 0.6</output>

<result>3/5 = around 0.6</result>