A project alone takes 10 days, B alone takes 15 days, and C alone takes 20 days. During the three-person collaboration, A asked for leave due to illness, and the project was completed in 6 days. How many days did A ask for sick leave?
一项工程，甲独做要10天，乙独做要15天，丙独做要20天．三人合做期间，甲因病请假，工程6天完工，问甲请了几天病假？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

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

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

<gadget id="calculator">(7/60) * 6</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 - (7/10)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(3/10) / (1/10)</gadget>
<output>3</output>

<gadget id="calculator">6 - 3</gadget>
<output>3</output>

<result>3</result>