(2/3)+(2/15)+(2/35)+...+(2/399).
(2/3)+(2/15)+(2/35)+…+(2/399)．
<gadget id="calculator">1 / 21</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">1 - (1/21)</gadget>
<output>20/21 = around 0.952381</output>

<result>20/21 = around 0.952381</result>