Three numbers p, p+1, p+3 are all prime numbers, what is the reciprocal of their reciprocal sum.
三个数p，p+1，p+3都是质数，它们的倒数和的倒数是多少．
<gadget id="calculator">30 / 31</gadget>
<output>30/31 = around 0.967742</output>

<result>30/31 = around 0.967742</result>