Answer
$a_{80} =-390$
Work Step by Step
The $n^{th}$ term of an arithmetic sequence is given by the formula:
$a_n = a_1 + (n-1)d (1)$
where $a_1 = \ First \ Term; \\ d = \ Common \ Difference$
We have: $a_1=5 \\ d=a_2-a_1=0-5=-5$
We will substitute the above data into formula (1) to obtain:
$a_n=5+(-5) (n-1) \implies a_n=5-5(n-1)$
In order to compute the $80th$ term, we need to plug in $80$ for $n$ into the above form to obtain:
$a_{80} = 5-5(80-1)=5-395$
So, $a_{80} =-390$
