If the same number is indicated in both places where the words "a number" appear, then the statement is true only when that number is -4 .
If "a number" and "a number" stand for two different numbers, then there are an infinite list of ordered pairs of numbers that make the statement true.
Because if you ask one student from each class you are getting a more accurate representation of the classes. If you were to ask one group you'd only get their opinion