You realize that an interview is not the same as writing a technical article, right? It should be a conversation between two potential coworkers, and, in that context, I think saying "average" instead of "mean" is totally fine.

Incidentally, "mean" doesn't quite cut it, if you want to be a stickler about it. There are several types of means: arithmetic, geometric, harmonic, etc. By that standard, you just failed the interview.