A project alone takes 15 days to complete, and B alone takes 20 days to complete. If A and B each do half of the whole project, how many days will it take to complete it?
一项工程，甲单独做需要15天完成，乙单独做需要20天完成，如果甲、乙各做整个工程的一半，共需要多少天可以完成？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/2) / (1/15)</gadget>
<output>15/2 = around 7.5</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/2) / (1/20)</gadget>
<output>10</output>

<gadget id="calculator">(15/2) + 10</gadget>
<output>35/2 = around 17.5</output>

<result>35/2 = around 17.5</result>