A project is done alone, A takes 10 days, B takes 15 days, how many days can the two teams cooperate to complete the project.
一项工程，单独做，甲要10天，乙要15天，两队合作，多少天能完成这项工程．
<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>