Don't shoot me, but I'm pretty sure that this is going to depend on how you log it. If you're the type of guy that uses each line in your logbook for each one way flight, then no, that 15 mile trip would not count because you'd likely written it on a different line (after your 50 mile trip to airport B)

If you will log the entire trip on one line in your logbook, then I'd say that any flying that you do in the same aircraft on the same day, after you landed 50 miles away, is all going to count as Cross country time.

No examiner ever questioned me when I did the latter.