Anyone who cannot cope with mathematics is not fully human. At best he is a tolerable subhuman who has learned to wear shoes, bathe, and not make messes in the house.
While this, of course, is an unnecessarily harsh thing to say about the Mike and Elly Pattersons of the world, there is something of a kernel of truth in it. Where Heinlein would deny them and Lynn the classification 'human being', I personally would insist that they not be called 'adult.'
This is because we are dealing with a woman who not only thinks it's funny and cute and charming that people have to pay her an allowance because she's too creative and special to be limited by such trivial and annoying concerns as having enough money to live on when it's clear to her that as soon as she gets any sort of cash, she's compelled to spend it immediately, she also thinks that being told to consider a future where the money dries up is something so scary and wrong, she thinks that the normal human response is to bawl like a five year old being told that she missed dessert time. There's a word for someone who refuses to look out for herself but insists that others keep her from slowly starving to death in a pile of her own filth and that word is "child."
This tendency Lynn has of thinking of herself as being too special a snowflake to be tied down by boring old numbers and tiresome logic and common sense manifests itself as her constantly being blindsided by things like the fact that Canada's insurance industry sees offering flood insurance as a license to ensure that by the end of business day, they'll go bankrupt as well as her thinking that having to pretend to run an independent bookstore is simply exchanging one form of thankless slavery for another. Trying to get her to see the truth would be like trying to make her understand the connection between her simply dumping the kids on other people at random and their habit of wanting to do the same.