The number of terms of the series $13 + 20 + 27 + 34 + \cdots + 111$ is —

Here $a = 13$, $d = 7$, and the last term is $111$, so $13 + (n-1)\cdot 7 = 111$ gives $(n-1)\cdot 7 = 98$, hence $n-1 = 14$ and $n = 15$.