Translation:I exercise twenty hours a week.
I was marked wrong for inserting the word "for" between "exercise" and "twenty". Ridiculous. On some items, Duolingo accepts a remarkably wide range of forms as correct, and on others, it is very narrow in what it accepts, as you could imagine a very bad teacher who teaches by rote but doesn't really understand the subject they are teaching at all. I doubt the designers of the AI are reading this forum. Oh well.
Hi uppergardiner, I also am not an English native speaker, and it was really frustrating the issue between do and make, but here is a hint... whenever you want to say something that you create or prepare like a machine or food (breakfast, lunch, dinner) you use (make); when you want to say that you did your homework, or did your dishes, laundry, etc. you use (do), exercise is an example of (do). You can't make exercise but instead you do exercise. Hope I had helped you! :D
I think in this case it's nor marked wrong because you choose make instead of do, but because in this instance we use exercise as the verb.
There are of course a frustrating number of exceptions, but we generally use make when the end product is a thing (make a cake, make a work of art) and do when it's more of an action (do gymnastics, do arithmetic). Don't ask me why it's make an effort, though.