A project alone takes 10 hours and B alone takes 15 hours. Now A and B are working together, how many hours can it be completed?
一项工程，甲独做要10小时，乙独做要15小时．现在甲乙合做，多少小时可以完成？
<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>