For a project, A and B jointly completed 1/2 of the whole project in 6 days, and A worked alone for 10 days, and B worked alone for 3 days to complete the whole project. Then, how many days will it take for B to complete this project alone?
一项工程，甲、乙合做了6天完成了全部工程的(1/2)，余下的由甲单独做10天，再由乙单独做3天，正好完成全部的工程．那么，这项工程由乙单独完成需要多少天？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/2) / 6</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 3</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/2) - (1/4)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">10 - 3</gadget>
<output>7</output>

<gadget id="calculator">(1/4) / 7</gadget>
<output>1/28 = around 0.035714</output>

<gadget id="calculator">(1/12) - (1/28)</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">1 / (1/21)</gadget>
<output>21</output>

<result>21</result>