A project can be completed in 3 days by A alone, and 6 days by B alone. How many days will the two work together? What is the correct formula
一项工程，甲独做3天可完，乙独做6天可完，两人合做几天完成？正确的计算式是多少
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/3) + (1/6)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / (1/2)</gadget>
<output>2</output>

<result>2</result>