Answer:
±320
Explanation:
You want the 4th term of the geometric progression that has first and third terms 5 and 80.
Common ratio
The general term of a geometric sequence is ...
an = a1·r^(n-1)
for first term a1 and common ratio r.
We are given the 1st and 3rd terms, so we can find the common ratio from ...
80 = 5·r^(3-1) . . . . . use a3 = 80, a1 = 5, n = 3
16 = r^2 . . . . . . divide by 5
±4 = r . . . . . . . . take the square root
Fourth term
The 4th term will be the product of the 3rd term and the common ratio:
a4 = a3·(±4) = 80·(±4) = ±320
The fourth term is -320 or +320.