Question 17.

General term = $38 + (n-1)17 = 17n + 21 = 17(n+1) + 4 = 17k + 4$

Each term is in the form of $17k + 4$

Least 3-digit number in the form of $17k + 4$ is at $k = 6$, i.e. $106$

Highest 3-digit number in the form of $17k + 4$ is at $k = 58$, i.e. $990$

$106, 123, 140, \ldots, 990$

$990 = 106 + 17(n-1)$

$n = 53$

Sum = $\frac{53}{2}(106 + 990) = 53 \times 548$

Avg = $53 \times \frac{548}{53}$ = 548