I knew that 420 was a code for cannabis, relating originally to the amount that could be purchased for the amount - $20 bought 4 of a measure.
Of course 42 is the answer to the meaning of life, the universe, and everything in the Hitchhiker's Guide to the Galaxy.
My theory is that Douglas Adams wouldn't have got away with 420 being the answer on the BBC or with the publisher, so it became '42'.
So the answer to the meaning of life, the universe, and everything is cannabis?