For a project, it takes 12 days for A to complete alone, and 9 days for B to complete alone. If A works for several days first, and then B alone takes 10 days in total, how many days does A do alone?
一件工程，甲单独完成要12天，乙单独完成需9天，若甲先做若干天，再乙单独完成共需10天，甲单独做了几天？
<gadget id="calculator">10 / 9</gadget>
<output>10/9 = around 1.111111</output>

<gadget id="calculator">(10/9) - 1</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">1 / 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">(1/9) - (1/12)</gadget>
<output>1/36 = around 0.027778</output>

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

<result>4</result>