Answer
$a_{80} =\dfrac{83}{2}=41.5$
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: $a_1=2 \\ d=a_2-a_1=\dfrac{5}{2}-2=\dfrac{1}{2}$
We will substitute the above data into formula (1) to obtain:
$a_n=2+(1/2) (n-1) \implies a_n=2+\dfrac{1}{2}(n-1)$
In order to compute the $80th$ term, we need to plug in $80$ for $n$ into the above form to obtain:
$a_{80} =2+\dfrac{1}{2}(80-1)=2+\dfrac{79}{2}$
So, $a_{80} =\dfrac{83}{2}=41.5$
