If I could work it out in my head, I wouldn't be a writer.
I found this rather interesting because after years of practice, amusing myself with day dreams whilst walking to school, I could create and store in memory a whole library of adventures. If I'd just wanted to do it for recreation, I would have stopped at that, but the reason I decided to become a writer was that I decided that the best of the day dreams were as good as some of the books I was reading. I therefore thought other people might find them interesting -- by this time I was about 14 and had a best friend at school who also wrote and we shared stuff. It was purely the urge to share ideas that made me want to write.
I still do all the "plot wrestling" in my head. I don't even write notes until the idea has firmed up sufficiently that I want to preserve that particular version. The notes, or even the first draft of a scene, might get superseded later, but it was always the best my writer-brain could do at the time.
I still do plotting in my head and usually while out on a walk. I love the buzz I get when the problem resolves and the right answer clicks into place. Which by total coincidence, I wrote about exactly a year ago. *g*