What are funny math problems?

One funny math problem is: "I am an odd number. Take away one letter and I become even. What number am I?" The answer is seven. The humor in this problem becomes apparent when the solver recognizes that the mathematics of odd and even numbers has been mixed with wordplay.

Another funny math problem is: "If I start with $10 and spend all but $3, how much do I have left?" The answer is $3; however, many people expect the $3 to be the amount spent and are fooled into saying "$7." An amusing mathematical pattern puzzle is: "What is the next letter in the following series? O T T F F S S E." The answer, N, is identified when the letters are recognized as simply the initial letters of the counting numbers.