There has to be evidence of their process for me to accept it as evidence of understanding/ability. I have made it clear to them that this is necessary. Their job is to convince me that they know what they’re doing. (But… I’m teaching HS Mathematics). So … I’d mark it wrong/incomplete. I’m also working on student understanding of consequences of their actions, so wouldn’t give them another opportunity on that exam. They would need to improve things on the next exam.