There is a manuscript, A alone finished typing in 4 days, B alone finished typing in 5 days. A typed this manuscript ((())/(())) every day, B typed this manuscript ((())/(())) every day, A and B worked together in one day to complete this ((())/(())) of the manuscript. So how many days will A and B play together to complete it?
有一份稿件，甲单独打4天打完，乙单独打5天打完．甲每天打这份稿件的((())/(()))，乙每天打这份稿件的((())/(()))，甲、乙两人合打一天要完成这份稿件的((())/(()))．那么甲、乙两人合打多少天完成．
<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 / (9/20)</gadget>
<output>20/9 = around 2.222222</output>

<result>20/9 = around 2.222222</result>