How much more is the fractional unit of the reciprocal of (3/7) than the original fractional unit.
(3/7)的倒数的分数单位比原来的分数单位多多少．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(1/3) - (1/7)</gadget>
<output>4/21 = around 0.190476</output>

<result>4/21 = around 0.190476</result>