What kind of Apple Mac did Arthur Dent have?

I like D.A.'s answer, but to be fair, there are certain contexts in which questions of this sort are not assumed trivial. I'm thinking specifically of Joyce's Ulysses; there's every indication that the author thought through every last detail of his fictional Dublin. The book demands that the reader negotiate her way through a thicket of of raw facts, and it has produced mountains of scholarship on seeming trifles, not all of it in the service of deep profundity, but some it quite startling in its cleverness and insight.

If my experience of growing up with Star Wars and reading about fans' notions of canonicity is any indicator, I'd expect that readers of SF and fantasy often have similar expectations of total world-building on the author's part, for better or for worse. There are thousands of "facts" which Star Wars fans "know" about the Star Wars universe, though that universe is not real, and none of those facts appear in the movies. (Sometimes those "facts" sprang into being to suit Kenner's needs.) Which is to say that, contra Adams, fictional worlds -- though not real -- often do have a sort of reality that extends beyond the author's primary text.

(The difference between my Star Wars example and Ulysses, though, is that the Ulysses "facts" are literally in the text or are inferred from it.)

A better answer: "What kind of Apple Mac do you imagine that Arthur had when you read the book? That's the kind he had."

The thing is, some authors _do_ imagine the world in more detail. Some of them are very visual, and when they're writing about Arthur Dent at his computer they're imagining it in enough detail that they could tell you the model that was in their head at the time.

It was an SE/30. By sheer coincidence, I have one too!

He had the Earth computer.

Or perhaps it was the other way around.

Simple. He had a Lisa.