I think the answer is 1,600 totals students taking metaphysics
Consecutive numbers differ by one, which means that if you start with a certain integer
, you obtain the next integer by adding one.
So, if you start with
, the three consecutive numbers are

The sum of these three numbers is

This number is divisible by 3 because you can write it as

Note that this is true in general: the sum of three consecutive numbers is always divisible by three, because it is three times the middle number.
She'd have to sell at least 15 bags to make her money back.