To complete a project, A works alone for 21 days and then B works alone for 12 days. If A and B cooperate for 14 days, the project can also be completed. How many days will it take B to complete the project alone?
要完成一个项目，甲单独做21天后再由乙单独做12天．如果甲、乙两人合作14天，也可以完成该项目．则乙单独完成这个项目需多少天．
<gadget id="calculator">1 / 14</gadget>
<output>1/14 = around 0.071429</output>

<gadget id="calculator">(1/14) * 12</gadget>
<output>6/7 = around 0.857143</output>

<gadget id="calculator">1 - (6/7)</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">21 - 12</gadget>
<output>9</output>

<gadget id="calculator">(1/7) / 9</gadget>
<output>1/63 = around 0.015873</output>

<gadget id="calculator">(1/14) - (1/63)</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">1 / (1/18)</gadget>
<output>18</output>

<result>18</result>