Answer
$a.\quad 42$
$b.\quad 2130$
Work Step by Step
The number of bricks is described as
$\left\{\begin{array}{l}
a_{1}=100\\
a_{n}=a_{n-1}-2
\end{array}\right.$, an arithmetic sequence with common difference $d=-2$.
$a.$
The n-th term is $a_{n}=a_{1}+(n-1)d$,
so the thirtieth is $a_{30}=100-2(29)=42$
$b.$
The sum $S_{n}$ of the first $n$ terms of $\left\{a_{n}\right\}$:
$S_{n}=\displaystyle \frac{n}{2}\left(a_{1}+a_{n}\right)$
$S_{30}=\displaystyle \frac{30}{2}(100+42)=15(142)=2130$
