A project alone needs 4 days to complete, and B completes (1/5) of the total amount every day. If two people work together in 2 days, how much is the project completed?
一项工程，甲单独做需4天完成，乙每天完成总数的(1/5)，两人合做2天，完成这项工程的多少．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

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

<gadget id="calculator">(1/4) + (1/5)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">(9/20) * 2</gadget>
<output>9/10 = around 0.9</output>

<result>9/10 = around 0.9</result>