For a fraction, if 3 is added to the numerator, the fraction is equal to (1/2), and if 1 is added to the denominator, the fraction is equal to (1/3), find the fraction.
一个分数，如果分子加3，这个分数等于(1/2)，如果分母加1，这个分数等于(1/3)，求这个分数．
<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">7 / 20</gadget>
<output>7/20 = around 0.35</output>

<result>7/20 = around 0.35</result>