smc

Let $a=1$. Our list is $\{1,2,3,4,5\}$ with an average of $15÷5=3$. Our next set starting with $3$ is $\{3,4,5,6,7\}$. Our average is $25÷5=5$. Therefore, we notice that $5=1+4$ which means that the answer is $\boxed{a+4}$.