project is completed in 20 days, B in 12 days, and C in 10 days. The actual number of days of cooperation between the three is an integer. A and C have worked on a total of 13 days. Find the number of days B worked.
一项工程甲20天做完，乙12天做完，丙10天做完，实际为三人合作天数都为整数，甲丙共计做了13天，求乙做的天数．
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 11</gadget>
<output>11/20 = around 0.55</output>

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

<gadget id="calculator">(1/10) * 2</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (11/20) - (1/5)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/4) / (1/12)</gadget>
<output>3</output>

<result>3</result>