What is the sum of all the simplest proper fractions whose denominator is 20?
分母是20的所有最简真分数的和是多少．
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">3 / 20</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">7 / 20</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">9 / 20</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">11 / 20</gadget>
<output>11/20 = around 0.55</output>

<gadget id="calculator">13 / 20</gadget>
<output>13/20 = around 0.65</output>

<gadget id="calculator">17 / 20</gadget>
<output>17/20 = around 0.85</output>

<gadget id="calculator">19 / 20</gadget>
<output>19/20 = around 0.95</output>

<gadget id="calculator">(1/20) + (3/20) + (7/20) + (9/20) + (11/20) + (13/20) + (17/20) + (19/20)</gadget>
<output>4</output>

<result>4</result>