An AP has 30 terms. Its first term is 12 and its last term is 128. Find the sum of all 30 terms.

With a and $l$ both known, use $S=\frac{n}{2}$$(a+l)=\frac{30}{2}$$(12+128)=15(140)=2100$. The value 2030 multiplies the average 70 by $(n-1)=29$ instead of by $n=30$ — the last-term formula already covers all $n$ terms. 2170 uses 31 terms. 4200 leaves out the factor $\frac{1}{2}$.