What is the sum of all the simplest proper fractions whose fraction unit is (1/7).
分数单位为(1/7)的所有最简真分数的和是多少．
<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">6 / 7</gadget>
<output>6/7 = around 0.857143</output>

<gadget id="calculator">(1/7) + (2/7) + (3/7) + (4/7) + (5/7) + (6/7)</gadget>
<output>3</output>

<result>3</result>