It takes 12 days for Xiaomeng and Xiaoli's mothers to knit a sweater together. It is known that Xiaomeng's mother knits 3 days and Xiaoli's mother 4 days. The amount of sweater is equal. How many days does Xiaomeng's mother alone need?
织一件毛衣，小蒙和小利两人的妈妈合织需12天，已知小蒙的妈妈织3天与小利的妈妈4天的毛衣量相等，小蒙的妈妈单独需多少天？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">4 / 3</gadget>
<output>4/3 = around 1.333333</output>

<gadget id="calculator">1 + (4/3)</gadget>
<output>7/3 = around 2.333333</output>

<gadget id="calculator">(1/12) / (7/3)</gadget>
<output>1/28 = around 0.035714</output>

<gadget id="calculator">(1/12) - (1/28)</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">1 / (1/21)</gadget>
<output>21</output>

<result>21</result>