Yes, six times nine equals fifty-four. Yes, six times nine equals 42 in base thirteen, and we don’t want to know about the implications that has on the number of fingers cavemen must have had.
Douglas has himself said:
: “The answer to this is very simple. It was a joke. It had to be a number, an ordinary, smallish number, and I chose that one. Binary representations, base thirteen, Tibetan monks are all complete nonsense. I sat at my desk, stared into the garden, and thought ‘42 will do’. I typed it out. End of story.”
Of course, this is not strictly true, as Douglas was using 42 before he wrote the Guide and admits in an interview in Douglas Adams' Guide To The Hitchhiker's Guide To The Galaxy that John Cleese had identified it as a funny number before he had.
(As an aside, Lewis Carroll was obsessed with the number 42).