For a document, it takes (3/4) hours for A to type alone, and (5/6) hours for B to type alone. After playing alone for (2/5) hours, how long will it take for two people to cooperate?
一份文件，甲单独打要用(3/4)小时，乙单独打要用(5/6)小时．甲单独打(2/5)小时后，两人合作，还要多长时间才能完成？
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 / (3/4)</gadget>
<output>4/3 = around 1.333333</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(4/3) * (2/5)</gadget>
<output>8/15 = around 0.533333</output>

<gadget id="calculator">1 - (8/15)</gadget>
<output>7/15 = around 0.466667</output>

<gadget id="calculator">5 / 6</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">1 / (5/6)</gadget>
<output>6/5 = around 1.2</output>

<gadget id="calculator">(4/3) + (6/5)</gadget>
<output>38/15 = around 2.533333</output>

<gadget id="calculator">(7/15) / (38/15)</gadget>
<output>7/38 = around 0.184211</output>

<result>7/38 = around 0.184211</result>