I am assuming you are trying to find the sum.
First, plug in n for 1, then n for 80 to get the following numbers:
5/6 and 163/6
As you go up the sequence, as n is increased by 1, then the next number is increased by 1/3
Use the arithmetic sequence formula to solve for the sum.
n * (a1 + a2)/2 where a1 is 1st number and a2 is last number.
80 * (5/6 + 163/6)/2
Simplify the expression to get 1120.