How many terms of G.P. `3,3^2,3^3,dotdotdot`are needed to give the sum 120?

Explanation

In the given GP,

`a = 3, r = 3`

`S_n = 120`

`:. a(r^n-1)/(r-1) = 120`

`=>3(r^n-1)/(3-1) = 120`

`=>3^n -1 = 80 => 3^n = 81`

`=> 3^n = 3^4`

`n = 4`