There is a fraction, if the denominator minus a certain number is equal to (7/8), if the denominator plus this number is equal to (12/17), what is the certain number?
有一个分数，若分母减去某数就等于(7/8)，若分母加上这个数就等于(12/17)，某数是多少？
<gadget id="calculator">12 / 17</gadget>
<output>12/17 = around 0.705882</output>

<gadget id="calculator">1 / (12/17)</gadget>
<output>17/12 = around 1.416667</output>

<gadget id="calculator">7 / 8</gadget>
<output>7/8 = around 0.875</output>

<gadget id="calculator">1 / (7/8)</gadget>
<output>8/7 = around 1.142857</output>

<gadget id="calculator">(17/12) - (8/7)</gadget>
<output>23/84 = around 0.27381</output>

<gadget id="calculator">(23/84) / 2</gadget>
<output>23/168 = around 0.136905</output>

<result>23/168 = around 0.136905</result>