Since,"if she has a work-out in the gym one evening, then the next evening she is twice as likely to have a swim as she is to have a work-out in the gym."

I would say that:

pr(gym|gym)=1/3

pr(swim|gym)=2/3

Since the sum of the two probabilities must equal 1 because it's stated that she does some sort of work out every evening.

Someone please correct me if I'm wrong.