A project can be completed in 6 days by A alone, 7 days by B alone, and 2 days jointly by two persons, how much is the project completed?
一项工程，甲独做6天完成，乙独做7天完成，两人合做2天，完成工程的多少．
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(1/6) + (1/7)</gadget>
<output>13/42 = around 0.309524</output>

<gadget id="calculator">2 * (13/42)</gadget>
<output>13/21 = around 0.619048</output>

<result>13/21 = around 0.619048</result>