The first term of an arithmetic progression is 8, and the sum of its first 16 terms is 608.

To find how many terms give a sum, put the sum into $S_n = \dfrac{n}{2}[2a + (n - 1)d]$ and solve the quadratic equation in $n$; keep the natural-number roots. To find $a$ or $d$, put in the known values and solve. From a formula for $S_n$, the $n$th term is $a_n = S_n - S_{n - 1}$, and $a_1 = S_1$.