A project can be completed by team A alone in 28 days, and the work efficiency of team B is 1/5 of that of team A. How many days can the two teams work together to complete half of the project?
一项工程，甲队单独做需28天完成，乙队的工作效率是甲队的(1/5)，两队合做多少天可以完成这项工程的一半？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 28</gadget>
<output>1/28 = around 0.035714</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/28) * (1/5)</gadget>
<output>1/140 = around 0.007143</output>

<gadget id="calculator">(1/28) + (1/140)</gadget>
<output>3/70 = around 0.042857</output>

<gadget id="calculator">(1/2) / (3/70)</gadget>
<output>35/3 = around 11.666667</output>

<result>35/3 = around 11.666667</result>