In Finish-to-finish (FF) dependency, the completion of the successor activity depends upon the completion of the predecessor activity. In simple terms, two activities can run concurrently, but one cannot finish before the other finishes.
Refer to the following blog post for detailed explanation:
http://www.deepfriedbrainproject.com/2010/01/critical-path-finish-to-finish-network.html
The attached PDF files provide the question and solution to the problem described in the blog post. Feel free to download the PDFs for your personal educational use.
Question:
Solution: