A piece of work can be completed in 10 days by A alone, and 15 days by B alone, how many days will A and B work together to complete.
一件工作，甲单独10天完成，乙单独15天完成，甲乙合做多少天完成．
<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>