yuk, yuk, yuk!

Teacher: What is 2k + k?
Student: 3000!

or

A mathematician and a stock broker go to the races to bet on horses. The broker suggests a bet of $10,000. That's too much for the mathematician's taste: First, he wants to understand the rules, have a look at the horses, etc.
"Don't worry", the broker says. "I know an empirical algorithm that allows me to find the number of the winning horse with absolute certainty."
This does not convince the mathematician.
"You are too theoretical!" the broker exclaims and puts his $10,000 on a horse.
The horse comes in first - making the broker even richer than he already is. The mathematician is baffled.
"What is your algorithm?" he wants to know.
"It's rather easy. I have two children, three and five years old. I add up their ages and bet on that number."
"But three plus five is eight - and that horse had number nine!"
"I told you that you're too theoretical! Didn't I just experimentally prove that my calculation is correct?!"