Both p and q are prime numbers, and 3p+5q=31, find the maximum value of pq. (Note: an represents the multiplication of n a)
p，q均为质数，且3p+5q=31，求pq的最大值．(注：an表示n个a相乘)
<gadget id="calculator">7 ** 2</gadget>
<output>49</output>

<result>49</result>