If A works in 2 days and B works in 5 days to complete the whole project (4/15), if A works in 5 days and B works in 2 days to complete the whole project (19/60), if A, How many days can B cooperate to complete.
一项工程，若甲做2天，乙做5天共完成全问工程的(4/15)，若甲做5天，乙做2天共完成全部工程的(19/60)，若甲、乙合作多少天可以完成．
<gadget id="calculator">4 / 15</gadget>
<output>4/15 = around 0.266667</output>

<gadget id="calculator">19 / 60</gadget>
<output>19/60 = around 0.316667</output>

<gadget id="calculator">(4/15) + (19/60)</gadget>
<output>7/12 = around 0.583333</output>

<gadget id="calculator">5 + 2</gadget>
<output>7</output>

<gadget id="calculator">(7/12) / 7</gadget>
<output>1/12 = around 0.083333</output>

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

<result>12</result>