An acre is 4840 sq yds. A square mile is 1760 x 1760 sq.yds.
so there are (1760 x 1760) divided by 4840 acres in a sq mile.
I think the answer is 640. Check my maths !
Thanks to all who answered The question was 'how many square acres' but must be a mistake. It is a New Years quiz in a local paper and I was down to the last two answers, just one now maybe I'll get it yet