If A, B, and C work together to complete a project in 4 days, if A and C work together in 5 days, how many days will it take if B alone completes it?
完成一项工程，如果甲乙丙三人合做4天完成，如果甲丙两人合做5天完成，如果让乙单独完成，需要多少天．
<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>1/20 = around 0.05</output>

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

<result>20</result>