A project is completed in 12 days alone, and B alone in 18 days. Now A works for a certain number of days, and then B alone completes the remaining tasks. In this way, it takes 16 days. How many days does A work first?
一件工程甲独做12天完成，乙独做18天完成，现在由甲先做若干天后，再由乙单独完成余下的任务，这样前后共用了16天，甲先做了多少天．
<gadget id="calculator">1 / 18</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">16 * (1/18)</gadget>
<output>8/9 = around 0.888889</output>

<gadget id="calculator">1 - (8/9)</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) - (1/18)</gadget>
<output>1/36 = around 0.027778</output>

<gadget id="calculator">(1/9) / (1/36)</gadget>
<output>4</output>

<result>4</result>