The nth term of an AP is $a_n=7-4n$. Find the sum of its first 20 terms.

$a_1=7-4=3$ and $a_{20}=7-80=-73$, so $S=\frac{n}{2}$$(a+l)=\frac{20}{2}$$(3-73)=10(-70)=-700$. The value -660 reads the constant 7 as the first term instead of computing $a_1=3$. 700 loses the sign, even though $d=-4$ makes the terms fall far below zero. -1400 forgets the factor $\frac{1}{2}$.