A project is completed by A alone in 12 days, and by B alone in 18 days. After A has done it for a certain number of days, B will finish the rest alone, so it takes 16 days. How many days did A do it first?
一件工程，甲单独做12天完成，乙单独做18天完成，现在甲做了若干天后，再由乙单独做完余下的部分，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">(1/18) * 16</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>