More importantly, for those who've been out of school for a while their math skills erode quickly. I'm having to reteach people basic linear algebra because they hadn't used it since their junior year of college 20 years ago and are now writing/maintaining programs that require it. Some of them were able to recall it with a bit of prompting, but for others it was a complete blank. For the former, the mechanics were still in their head but the terms were gone and needed to be reconnected, for the latter they aren't recalling either parts. But both groups are picking it back up quickly, they don't need a full semester of retraining, just a few sessions and then a lot of practice/application.