Copying a formula down a row that references different excel file

NEED HELP,

I need help my eyes are about to fall out.

I am trying to copy a formula down a row that references a another row in a different excel document. I would this there is an easy fix I am just not sure how to do it.

Attached is the document that I am working on any help would help.

I want to be able to drag the formula in C15 down to update the reference to the cell numbers with out having to manually enter the formula for each cell.

Thank you for the help

Gil