Answer: 500, or actually, somewhere between 450 and 550.
Explanation:
5/7 × 700 = 500
This isn't actually true unless someone actually counted the students. And if they did, the result wouldn't have been such a round number, so 5/7 is rounded
A better answer would be to interpret 5/7 as 4.5/7 to 5.5/7, or 450 to 550.