The principal sits in on a class to see how things are going. In the middle, he interrupts the teacher and asks if he can ask the class a question. The teacher obliges, and the principal asks the pupils: "This classroom is 10 meters long and 20 feet wide. How old am I?". No one answers, so he repeats the question and adds, "Are you all stupid or something?!". Again, silence.
Then little Tommy in the back row raises his hand. "Yes?", says the principal. Tommy says: "You are 54 years old!". "Correct," says the principal, "and would you kindly explain to the rest of your dim classmates how you figured that out?".
Tommy says "Certainly Sir. My Mother has a brother whose 27. And she always says that he's half-crazy!"