Sets Practice Test 1

Clearly, the number of elements in A ∪ B will be minimum when A ⊂ B. Hence, the minimum number of elements in A ∪ B is
{1, 2, 3, 4} ∪ {1, 2, 3, 4, 5, 6, 7}
= {1, 2, 3, 4, 5, 6, 7} same as the number of elements in B, that is 7.