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.
Severe cataracts?