Sum of a G.P. series is represented by the formula,  when r>1. ‘Sn’ represents the sum of the G.P. series upto n^th terms, ‘a’ represents the first term, ‘r’ represents the common ratio and ‘n’ represents the number of terms.

Here, a = 2

r = (ratio between the n term and n-1 term) 6 ÷ 2 = 3 Sn = 728

∴ 6 terms must be taken to reach the desired answer.