How many terms are there in the GP 5, 20, 80, 320........... 20480?

Answer: Option E

Common ratio, r = $$\frac{20}{5}$$ = 4
Last term or n^th term of GP = ar^{n - 1}
20480 = 5 × (4^{n - 1})
Or, 4^{n - 1} = $$\frac{20480}{5}$$  = 4⁶
So, comparing the power,
Thus, n - 1 = 6
Or, n = 7
Number of terms = 7