I tried this in university. I knew the right answer but not how to get it, so I made a "mistake" in the calcs. It was circled with "something's not right". No marks.
Conversely, when I was in college I once did all of the work right for a math problem that took two whole pages to do. And then I copied the wrong number to the answer line. But they saw my work and saw the right answer written down, so they only took like one point off of a 15 point problem or something like that.
I did the same. Forgot the formula but I explained how I would have solved the problem if I rememberd the formula. I think I got like 2/10 on the question lol
We had a formula once that you could aproach from two ways. So I went backwards once I couldn't figure out how to continue. In the end I still didn't know how to get from x to y but this was a small enough step the corrector would know surely.