42 Is the Answer! What Was the Question?

Douglas Adams was asked many times during his career why he chose the number 42. Many theories were proposed, but he rejected them all.

On November 3, 1993, he gave an answer on alt.fan.douglas-adams:
"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."

Adams described his choice as "a completely ordinary number, a number not just divisible by two but also six and seven. In fact it's the sort of number that you could without any fear introduce to your parents."

Look here Tinas universum or here Wikipedia.