A project can be completed in 12 days by A alone, and completed in 4 days by B alone. If A works for several days first, then B will continue to do the rest of the project by himself until the whole project is completed. In this way, it will take 6 days in total, and A will do it first. How many days?
一件工程，甲独做12天完成，乙独做4天完成，若甲先做若干天后，由乙接着单独做余下的工程，直至完成全部工程，这样前后一共用了6天，甲先做了多少天？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/4) * 6</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">(3/2) - 1</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">(1/4) - (1/12)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/2) / (1/6)</gadget>
<output>3</output>

<result>3</result>