Extra element theorem

Extra element theorem is a way of deciding if two sets of numbers are the same. It works by seeing if one set of numbers has an extra element (or number) in it that the other does not.

For example:

Set A: 1, 4, 7
Set B: 1, 3, 4

Set A has an extra element (7) that Set B does not have, so according to the extra element theorem, Set A and Set B are not the same.