A doctor is sent to a nursing home to test the minds and memories of the residents. To save time, she interviews them in groups of three. The first group she meets with consists of three men.
Turning to the first one, she asks, “What’s nine times thirteen?”
“That would be four hundred and six,” the man replies.
Without giving any indication that his answer is wrong, she turns to the second man. “What do you think, sir? What’s nine times thirteen?”
“That’s easy,” he says. “It’s Thursday.”
She turns to the third man and says, “Nine times thirteen?”
He answers immediately. “One hundred seventeen.”
“Excellent,” says the doctor. “How did you get it so quickly?”
“Simple,” he says. “I just subtracted four hundred and six from Thursday.”