Let n be the number of terms we needed. Here a = 3, r = 3, Sn = 120
We know Sn =

a(rn−1)

r−1

, if |r| > 1
Then 120 = 3

(3n−1)

3−1

⇒ 120 =

3

2

(3n−1)
⇒ 80 = 3n = 81 ⇒ 3n = (3)4
∴ n = 4
Hence, 4 terms are needed.