It takes A 4 days and B 5 days to complete a job alone, how many days can be completed by cooperation (1/3).
单独完成一项工作，甲要4天，乙要5天，合作多少天，能完成这项工作的(1/3)．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<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">(1/3) / (9/20)</gadget>
<output>20/27 = around 0.740741</output>

<result>20/27 = around 0.740741</result>