arithmetic sequence??

For an arithmetic sequence   Sn = n/2 (a1+an)

208 = 3/2(10+a3)

a3=128.66

10              a2                  128.66                (128.66+10)/2 = D  = 59.33

10        10+59.33         10 + 59.33+ 59.33

10         69.33                128.66

an then becomes       an= 10 + 59.33(n-1)          or     an = 59.33n - 49.33