This is an arithmetic series
The formula for an arithmetic series is

where,
an is the nth term, in this example it's the 80th term
a1 is the first term, in this example it's 16
n is the term position, n = 80 in this case
and, d is the common difference between terms, in this example, this is 5 because: 31-26= 5, 26-21=5 and so on
Now, let's replace the above

Thus, the 80th term is 411