If A alone takes 10 days to complete a project, and B alone takes 15 days to complete it, how many days will it take to complete the project if they work together?
一项工程，甲单独做要10天完成，乙单独做要15天完成，如果两人合做，需多少天完成这项工程．
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">(1/10) + (1/15)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / (1/6)</gadget>
<output>6</output>

<result>6</result>