Let n be the number of terms we needed. Here a = 3, r = 3, $S_n$ = 120 We know $S_n$ = $\frac{a(r^n-1)}{r-1}$ , if |r| > 1 Then 120 = 3 $\frac{3^n-1}{3-1}$ ⇒ 120 = $\frac{3}{2}(3^n-1)$ ⇒ 80 = $3^n$ = 81 ⇒ $3^n$ = $(3)^4$ ∴ n = 4 Hence, 4 terms are needed.