Sets that contain exactly the same elements.
Property
Two sets X and Y are equal if and only if: A ∩ B = A, A ∩ B = B, A ∪ B = A and A ∪ B = B.
Example
The set X of whole numbers under 30 that are multiples of 5 and the set Y of whole numbers under 30 with a ones digit that is 0 or 5 are equal sets.
Since X = {0, 5, 10, 15, 25} and Y = {0, 5, 10, 15, 25}, therefore: X = Y.