Until I met my MIL, I would tell people that I had never met anyone who is truly stupid, although I prefer the term simple-minded. She takes just about everything at face value, and seems to be unable to think deeply about anything.
So if I look up stupid in the dictionary, I’ll find a picture of your MIL? :-)
When my husband was in 4th grade he told his mother she was a mathematical moron. Sad to say, he was correct. She can’t add more than 2 numbers and when she writes down addresses or phone numbers someone has to check them because they are never correct. She gets numbers out of place and confused constantly. She graduated from University of Texas in 1954 with a degree in Home Ec/Early Childhood Education as well as what she really went there for, her MRS degree.
Her husband is a former NASA engineer. No joke a rocket scientist. They’re in their 70’s and he has absolutely no patience with her math disability.