A project alone takes 20 days to complete, and B alone takes 30 days to complete. Now A will do the work for 6 days first, and B will finish the rest by himself. How many days did B do it?
一项工程，甲单独做要20天完成，乙单独做要30天完成．现在甲先做6天后，剩下的由乙单独做完．乙做了多少天？
<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 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(7/10) / (1/30)</gadget>
<output>21</output>

<result>21</result>