fraction after division is (6/7), if the sum of the numerator and denominator before division is equal to 39, what is the fraction before division?
一个分数约分后是(6/7)，如果约分前的分子与分母的和等于39，约分前的分数是多少？
<gadget id="calculator">6 + 7</gadget>
<output>13</output>

<gadget id="calculator">6 / 13</gadget>
<output>6/13 = around 0.461538</output>

<gadget id="calculator">7 / 13</gadget>
<output>7/13 = around 0.538462</output>

<gadget id="calculator">39 * (7/13)</gadget>
<output>21</output>

<gadget id="calculator">(6/13) / 21</gadget>
<output>2/91 = around 0.021978</output>

<gadget id="calculator">39 * (2/91)</gadget>
<output>6/7 = around 0.857143</output>

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