(1+(1/7)+(1/9)+(1/11))*((1/7)+(1/9)+(1/11)+(1/21))-(1 +(1/7)+(1/9)+(1/11)+(1/21))*((1/7)+(1/9)+(1/11)).
(1+(1/7)+(1/9)+(1/11))*((1/7)+(1/9)+(1/11)+(1/21))-(1+(1/7)+(1/9)+(1/11)+(1/21))*((1/7)+(1/9)+(1/11))．
<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 / 11</gadget>
<output>1/11 = around 0.090909</output>

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

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

<gadget id="calculator">11 / 1</gadget>
<output>11</output>

<gadget id="calculator">1 + 7 + 9 + 11</gadget>
<output>28</output>

<gadget id="calculator">21 / 1</gadget>
<output>21</output>

<gadget id="calculator">7 + 9 + 11 + 21</gadget>
<output>48</output>

<gadget id="calculator">28 * 48</gadget>
<output>1_344</output>

<gadget id="calculator">1 + 7 + 9 + 11 + 21</gadget>
<output>49</output>

<gadget id="calculator">7 + 9 + 11</gadget>
<output>27</output>

<gadget id="calculator">49 * 27</gadget>
<output>1_323</output>

<gadget id="calculator">1_344 - 1_323</gadget>
<output>21</output>

<gadget id="calculator">1 / 21</gadget>
<output>1/21 = around 0.047619</output>

<gadget id="calculator">(1/7) + (1/9) + (1/11) + (1/21)</gadget>
<output>272/693 = around 0.392496</output>

<gadget id="calculator">(1/7) + (1/9) + (1/11)</gadget>
<output>239/693 = around 0.344877</output>

<gadget id="calculator">(272/693) - (239/693)</gadget>
<output>1/21 = around 0.047619</output>

<result>1/21 = around 0.047619</result>