If A alone takes 1 hour to do a job and B alone takes (4/5) hours, how many hours will it take for both of them to do it together?
一项工作，甲独做要1小时，乙独做要(4/5)小时，两人合做要多少小时完成．
<gadget id="calculator">1 / 1</gadget>
<output>1</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 / (4/5)</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">1 + (5/4)</gadget>
<output>9/4 = around 2.25</output>

<gadget id="calculator">1 / (9/4)</gadget>
<output>4/9 = around 0.444444</output>

<result>4/9 = around 0.444444</result>