What is the difference between the fractional unit of (6/7) and the fractional unit of (7/9).
(6/7)的分数单位与(7/9)的分数单位相差多少．
<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">(1/7) - (1/9)</gadget>
<output>2/63 = around 0.031746</output>

<result>2/63 = around 0.031746</result>