Answer:
a(4) = 15/4
Explanation:
The general formula for a geometric series with common ratio r and first term a(1) is
a(n) = a(1)*r^(n - 1)
and so, if a(1) = 30 and r = 1/2, we have
a(n) = 30*(1/2)^(n - 1)
and the fourth term is then:
a(4) = 30*(1/2)^(4 - 1) = 30/8 = 15/4