For a true fraction, the sum of the numerator and the denominator is 40. After converting it into the simplest fraction, the sum of the numerator and the denominator is 10. What is the original fraction?
一个真分数，分子与分母的和是40，化成最简分数后，分子与分母的和是10，原来的分数是多少
<gadget id="calculator">12 / 28</gadget>
<output>3/7 = around 0.428571</output>

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