A project is completed in 13 days by the cooperation of A, B, and C. If C rests for 2 days, B will have to do 4 more days, or A and B will work together to do 1 more day. How many days will the project be completed by B alone.
一项工程，甲、乙、丙三人合作13天完成，如丙休息2天，乙就要多做4天，或由甲、乙两人合做多做1天．这项工程由乙独做多少天完成．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">3 + 1 + 2</gadget>
<output>6</output>

<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">13 / (1/6)</gadget>
<output>78</output>

<result>78</result>