project, A and B cooperate to complete in 6 days, A alone takes 15 days, B alone how many days.
一项工程，甲、乙合作6天完成，甲单独做需15天，乙队单独做需多少天．
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

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

<gadget id="calculator">(1/6) - (1/15)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / (1/10)</gadget>
<output>10</output>

<result>10</result>