For a fraction, the sum of the numerator and the denominator is 21. After dividing it, the result is (3/4). What is the original fraction?
一个分数，分子与分母的和是21，约分后得(3/4)，原来这个分数是多少？
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">21 * (3/7)</gadget>
<output>9</output>

<gadget id="calculator">9 / 21</gadget>
<output>3/7 = around 0.428571</output>

<result>3/7 = around 0.428571</result>