9 W

From an interview with novelist Harry Mathews in the Paris Review (Spring 2007):

Can I tell you a joke? What is the question to which the answer is 9 W?

If you give up, you can see the question by highlighting the seemingly empty space that follows these words:

Mr. Wagner, do you write your name with a V?

New York and New Jersey commuters can think of other questions whose answer is 9W.