These guys can do math in their heads, but not on paper, and not much else either. Brain damage can be extremely specific, or extremely broad. There are people who only see the left portion of something, and so when they eat, they eat half of the plate of food, because that's all they can see, spin around, eat half the remainder, spin again, and eat half of what yet remains, and then stop. There are people who can remember every single minute event of their entire lives. There are people who can't remember what happened two minutes ago.
This issue isn't about being too dumb to answer a question and "earn" a prize. The question is to change it from a lottery, which requires a license and lottery number, to a prize, which does not have such requirements. She can probably lead an otherwise quite normal life, even if she can't do 6 * 8 - 5 + 9.
As far as I can tell, the only thing she's guilty of is being too honest to get her husband or kids to do the question for her.