2015 AMC 10A Problem 7

Recall that the $n$th term of an arithmetic sequence is $a + d(n - 1),$ where $a$ is the first term and $d$ is the common difference. For us, $a = 13$ and $d = 3.$ Plugging these in, we get that $$\begin{aligned} 73 &= 13 + 3(n - 1) \\ 20&= n - 1 \\ n &= 21. \end{aligned}$$ Thus, B is the correct answer.