There is a fraction whose numerator is 13 smaller than the denominator. If 18 is added to the numerator and 21 to the denominator, the value of the fraction remains the same. What is the original fraction?
有一个分数，分子比分母小13，如果分子加18，分母加21，分数值不变，原分数是多少？
<gadget id="calculator">78 + 13</gadget>
<output>91</output>

<gadget id="calculator">78 / 91</gadget>
<output>6/7 = around 0.857143</output>

<result>6/7 = around 0.857143</result>