The two formulas you need to know:
Find any term in an arithmetic sequence: An = A1 + (n-1)d
where A1 = is the first term, n is the number in the sequence, and d is the common difference.
Find the sum of an arithmetic sequence: Sn = 0.5n(A1+An).
To solve d, we need to know n for the An=67. To do this we can use the sum equation:
Sn = 0.5n(A1+An)
680 = 0.5n(10+67)
n = (2*680)/(10+67) = 17.66
