A fraction value is (3/4), the sum of the numerator and the denominator is 21, what is the fraction.
一个分数值是(3/4)，分子与分母的和是21，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">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">21 * (4/7)</gadget>
<output>12</output>

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

<gadget id="calculator">21 * (1/28)</gadget>
<output>3/4 = around 0.75</output>

<result>3/4 = around 0.75</result>