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

Explanation:-

Common ratio, r = 20/5 = 4;
Last term or n^th term of GP = ar^n-1.
20480 = 5*(4^n-1);
Or, 4^n-1 = 20480/5 = 4⁸;
So, comparing the power,
Thus, n-1 = 8;
Or, n = 7;
Number of terms = 7.