It was definitely a dorky question, and a glaring sign of the interviewer's lack of maturity to blithely burn through your precious time in this manner.

But if you're going to play Google's game, this what you're going to get apparently. So what was your answer? One guesses that they weren't so much interested in a correct answer (since it's a bullshit question anyway) but that you were willing to give it a college try (and reference some concepts about limits, and perhaps the geometric layout of this "file system" in the Doctor Who universe they were apparently hoping to roll out this system in, the speed of light, etc).

In order to, you know, pass their dipshit filter.