A project alone takes 12 days to complete and B takes 9 days to complete alone. If A does it for a few days first and then B continues to do it, and it takes 10 days to complete it, may I ask: How many days did A do it alone?
一项工程，甲单独完成需12天，乙单独完成需9天．若甲先做若干天后乙接着做，共用10天完成，请问：甲单独做了几天？
<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">(1/9) * 10</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 / 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>