A project can be completed in 20 days by A alone, and 40 days by B alone. How many days will it take for A and B to cooperate?
一项工程，甲单独做20天完成，乙单独做40天完成，如果甲、乙合作几天才能完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">(1/20) + (1/40)</gadget>
<output>3/40 = around 0.075</output>

<gadget id="calculator">1 / (3/40)</gadget>
<output>40/3 = around 13.333333</output>

<result>40/3 = around 13.333333</result>