What is the sum of all the proper fractions whose denominator is 15?
分母是15的所有的真分数之和是多少．
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">2 / 15</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">4 / 15</gadget>
<output>4/15 = around 0.266667</output>

<gadget id="calculator">7 / 15</gadget>
<output>7/15 = around 0.466667</output>

<gadget id="calculator">8 / 15</gadget>
<output>8/15 = around 0.533333</output>

<gadget id="calculator">11 / 15</gadget>
<output>11/15 = around 0.733333</output>

<gadget id="calculator">13 / 15</gadget>
<output>13/15 = around 0.866667</output>

<gadget id="calculator">14 / 15</gadget>
<output>14/15 = around 0.933333</output>

<gadget id="calculator">(1/15) + (2/15) + (4/15) + (7/15) + (8/15) + (11/15) + (13/15) + (14/15)</gadget>
<output>4</output>

<result>4</result>