project is done alone, B takes 2 days less than A, B completes (1/5) of this project every day, how many days can A and B cooperate to complete this project (2/3).
一项工程，单独做，乙比甲少用2天，乙每天完成这项工程的(1/5)，甲、乙合作多少天可以完成这项工程的(2/3)．
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">5 + 2</gadget>
<output>7</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(1/5) + (1/7)</gadget>
<output>12/35 = around 0.342857</output>

<gadget id="calculator">(2/3) / (12/35)</gadget>
<output>35/18 = around 1.944444</output>

<result>35/18 = around 1.944444</result>