"Mangiamo una mela."
It doesn't have to be only one apple. 'We eat an apple' can indicate that every person eats one.
Hmm, I think this is a rather complex topic. In my opinion, that sounds incorrect, but that's of course just an opinion. I would only say "We eat apples." or "Each of us eats an apple".
I have the same problem. The way she's reading it, I have to slow it down a few times just to make sure she's actually saying "una"
Hehe, why did they put "We eat an man"? as one of the multiple choices? :P It's obviously wrong, since "an" is used before words beginning in vowels. But whatevs.