project is done by A for a hour and then A and B cooperate. When it is completed, A has done (4/5) of the project; project (2/5). This project is completed by A alone in 10 hours, and how many hours it will be completed by A and B at the same time.
一项工程，由甲先做a小时后甲乙二人合作，完成时甲做了这项工程的(4/5)；如果由乙先做a小时后甲乙二人合作，完成时甲做了这项工程的(2/5)．这项工程由甲单独做10小时完成，由甲乙二人同时开工合作多少小时完成．
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(4/5) * 10</gadget>
<output>8</output>

<gadget id="calculator">8 - 5</gadget>
<output>3</output>

<gadget id="calculator">(1/5) / 3</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / (1/15)</gadget>
<output>15</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/10) + (1/15)</gadget>
<output>1/6 = around 0.166667</output>

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

<result>6</result>