For the arithmetic series 13+19+...

a. find tsub27
b. find Ssub27
c. if tsubn = 445, find n

a=13

d=6
T27 = 13+26*6 = 169
S27 = 27/2 (2*13 + 26*6) = 2457

445 = 13 + (n-1)6
n=73