If we flew over 500 miles of land and then flew 'over 2000 miles of water' our trip would have taken us 2500 miles, of which only 2000 miles were flown over water.
To answer your questions, it's possible that answer 'a' could be exactly 2000 miles (although, it is not a certainty if we also flew over a portion of land).
Question 'b' is likely to mean 'more than' instead of 'exactly'.
In either case, it is highly unlikely that either flight would be EXACTLY 2000 miles!!!!
- For Teachers