What is the sum of all the proper fractions whose denominator is 3?
分母是3的所有真分数的和是多少．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

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

<result>1</result>