For a manuscript, A typed (2/3) hours alone to type (2/9) of this manuscript, and B completed it in 4 hours alone, how many hours did A and B cooperate to complete it?
一份稿件，甲单独打(2/3)小时打了这份稿件的(2/9)，乙单独打4小时完成，甲、乙两人合作几小时完成？
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">2 / 9</gadget>
<output>2/9 = around 0.222222</output>

<gadget id="calculator">(2/3) / (2/9)</gadget>
<output>3</output>

<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">(1/3) + (1/4)</gadget>
<output>7/12 = around 0.583333</output>

<gadget id="calculator">1 / (7/12)</gadget>
<output>12/7 = around 1.714286</output>

<result>12/7 = around 1.714286</result>