I think the OP's posted the questions rather poorly but really the question has nothing to do with determining correct exposure. It only asks the difference in stops for 2 camera settings.

In the first post is really asking between f/16@1/100@ISO100 and f/2.8@1/50@ISO800 how many stops difference are there?
In the second post is really asking between f/16@1/100@ISO100 and f/2.8@1/50@ISO640 how many stops different?

That's all has nothing to do with getting proper exposure and that is why I think it's an academic question and not something the OP wanted to know.