Answer
See below.
Work Step by Step
a) The nth term of an arithmetic sequence can be obtained by the following formula: $a_n=a_1+(n-1)d$, where $a_1$ is the first term and $d$ is the common difference.
Hence here: $d=-3,n=25,a_1=80,a_{25}=80+(25-1)(-3)=80-72=8$
b) The sum of the first $n$ terms of an arithmetic sequence can be obtained by the following formula: $\frac{n(a_1+a_n)}{2},$ where $a_1$ is the first term, $a_n$ is the nth term and $n$ is the number of terms.
Hence here the sum: $\frac{n(a_1+a_n)}{2}=\frac{25(80+8)}{2}=1100$
