The formula for a geometric sequence is an = a1 * r^n-1. For this problem, our formula can be an = 1 * 5^n-1. a1 is our first term, and r is the common ratio.
To check our formula, we can find the 5th term. a5 = 1 * 5^5-1. This gives us 625. If we look at our last term provided, the 4th term, 125 * 5 is 625.