I found a similar question in
"Data Communications and Networking By Behrouz A.Forouzan 4th Edition Chapter 11 Data Link Control, Exercise Question 30, 31, 32"

And below is the solution given in the Solution Manual.

In all the cases, they have calculated the worst case time (waiting for the acknowledgment of the whole window).
So if found using the best case, then the answer will be different.
So which approach to use is a confusion.