The number of term in a geometry progression

Hint : You can find the number of term in an arithmetic progression if you know the last term

Find the number of terms for each of the following geometric progressions.
(a) 2, 4, 8,……, 8192
(b) 1 4 , 1 6 , 1 9 ,..., 16 729
(c) 1 2 ,1,2,...,64