Answer:

Explanation:
You want the sum 15 -3 +3/5 -3/25 written using sigma notation.
Sigma notation
Sigma notation makes use of the expression for a general term, using the term index number as a variable. Here the terms are considered to be 1 through 4, so the number above the sigma symbol (∑) is 4.
General term
The series is a geometric series with first term 15 and common ratio -1/5. The general term is written as ...
an = a1 +r^(n-1)
where a1 is the first term, r is the common ratio, and n is the term index.
For our sigma expression, we will use i as the term index, so we have ...

__
Additional comment
The factor 15 can be brought outside the sigma expression. The sum of these four terms is 12.48, or 12 12/25.
<95141404393>