The sum of the numerator and denominator of a fraction is 31. If 5 is subtracted from both the numerator and the denominator, the fraction is (2/5). What is the original fraction?
一个分数的分子与分母的和是31，若分子与分母都减去5，约分后是(2/5)，原分数是多少．
<gadget id="calculator">31 - 5 - 5</gadget>
<output>21</output>

<gadget id="calculator">2 + 5</gadget>
<output>7</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">21 * (2/7)</gadget>
<output>6</output>

<gadget id="calculator">6 + 5</gadget>
<output>11</output>

<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">21 * (5/7)</gadget>
<output>15</output>

<gadget id="calculator">15 + 5</gadget>
<output>20</output>

<gadget id="calculator">11 / 20</gadget>
<output>11/20 = around 0.55</output>

<result>11/20 = around 0.55</result>