Sum of the First n Terms of a Geometric Sequence
Given a geometric sequence (or series) with a first-term a1 and common ratio r, the sum of the first n terms is given by:

We are given the series:

Before calculating the required sum, we need to find the common ratio. It's defined as the division of two consecutive terms. For example, using the first two terms:

The first term is a1 =120. Now apply the formula:

Operating:

Calculating:
