What is the sum of all the simplest proper fractions whose fraction unit is (1/5).
分数单位是(1/5)的所有最简真分数的和是多少．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

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

<result>2</result>