What is the sum of the first eight terms of a geometric series whose first term is 3 and whose common ratio is 1/2?

There is a formula...
Sn = a1 (1 - r^n) / (1 - r)
a1 = first term = 3
n = number of terms = 8
r = common ratio = 1/2

now we sub
S(8) = 3(1 - 1/2^8) / (1 - 1/2)
S(8) = 3(1 - (1/2^8) / (1/2)
S(8) = 3(1 - 1/256) / (1/2)
S(8) = 3 (256/256 - 1/256) / (1/2)
S(8) = 3(255/256) / (1/2)
S(8) = (765/256) / (1/2)
S(8) = 765/256 * 2/1
S(8) = 1530/256
S(8) = 765/128 or 5 125/128 or 5.9765625