A project is done alone, A takes 20 days, B takes 15 days, and the two teams cooperate, how many days can they complete the project (7/10).
一项工程，单独做，甲要20天，乙要15天，两队合作，多少天能完成这项工程的(7/10)．
<gadget id="calculator">7 / 10</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

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

<gadget id="calculator">(1/20) + (1/15)</gadget>
<output>7/60 = around 0.116667</output>

<gadget id="calculator">(7/10) / (7/60)</gadget>
<output>6</output>

<result>6</result>