A and B work together to complete a project in 6 days, and A completes it every day (1/9). If B alone does this project, how many days will it take?
甲、乙两人合作某项工程，6天可完成，甲每天完成(1/9)，如果乙单独做这项工程需几天？
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">(1/6) - (1/9)</gadget>
<output>1/18 = around 0.055556</output>

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

<result>18</result>