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.
the way it was worded was difficult to understand
but the end answer isn't very difficult to figure out ... i think ?