How many terms are there in the GP 5, 20, 80, 320, ...,

Let term = l = ar^{n - 1} a = 5 and l = 20480
r = 20/5 = 4
∴ 20480 = 5 x (4)^n-1
(4)^n-1 = 20480/5 = 4096 = (4)⁶
n - 1 = 6
∴ n = 7