The three numbers a, a+1, and a+3 are all prime numbers. What is the sum of their reciprocals?
三个数a、a+1、a+3、都是质数，它们倒数和是多少．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/2) + (1/3) + (1/5)</gadget>
<output>31/30 = around 1.033333</output>

<result>31/30 = around 1.033333</result>