"The boy sits."
The ratio is, in my experience, somewhere between 1/5 and 1/3 original language to new language exercises for all duolingo courses, and does not change. The exercises as you see them for each lesson are created by the machine "at random" from a pool of bi-directional translations supplied by the creators of the course for that unit. The ratio is pre- programmed in the system, and neither that nor the concrete types of exercises that the machine makes out of your sentence pool can be influenced by the course creators, apart from disabling certain exercise types for certain sentences.
That is to say, there are no "english-to-swedish" or "swedish-to-english" sentences, every single sentence can appear in either direction unless that has been explicitely disabled by the translator, say because the reverse translation will never arrive at the original sentence or similar - but the latter is very uncommon (at least in the courses I've seen the insides of).
Does someone know how much is the ratio of Swedish to English vs English to Swedish sentences? Because it seems to me that there are more Swedish to English sentences and less English to Swedish sentences like this one, so I'm wondering does that change as we progress further into the tree or?