Geometrical progression is given by the following formula:
an+1=an*q, where q is the common ratio. From this formula, you can obtain another one, relating the first term of progression to the arbitrary one,
So, for the sixth term in your case, we need to determine q and a1. a1=625; q=125/625, ratio of two successive terms, q=1/5.
Now, for the sum of N terms, from the first to the Nth one.
Let us multiply and divide this expression by (q-1). We will obtain the following:
Thus, for the sum of 15 terms we will get: