Diagnosing Jokes

“Alright, I have your Diagnosis,” The doctor says to the patient

The patient says, “Make it quick. I don’t got all day.”

“How did you know?”

"What’s 9 plus 9?”
“12”.
“What’s 8 and 8?”
“10”.
The doctor shook his head. “Very interesting. What about 6 times 5?”
The man thought for a second, and answered “1E”.
“Aha, I’ve figured it out!” The doctor said. “Somebody’s clearly put a hex on you.”

Not hard