A road, team A alone repaired it for 15 days, team B alone repaired it for 20 days, team A was transferred away, and team B repaired it in another 6 days, how many days did they cooperate?
一条路，甲队单独修15天，乙队单独修20天，甲队调走，乙队又修6天完工，他们合作几天？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 6</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (3/10)</gadget>
<output>7/10 = around 0.7</output>

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

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

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

<result>6</result>