Find the sum of the first 20 terms of the arithmetic progression $2, 5, 8, 11, \ldots$

Here $a = 2$, $d = 3$ and $n = 20$. Using $S_n = \dfrac{n}{2}[2a + (n-1)d]$: $S_{20} = 10[4 + 19 \times 3] = 10 \times 61 = 610$. Forgetting to halve $n$ gives 1220.