Sets A and B have 13 elements in common. The total of their union is 107. But this 107 only accounts once for the 13 elements in common. Thus, the two sets actually possess a total of 107 + 13 = 120 elements between them. Finally, 120 - 69 = 51. The number of elements in set B is 51.
QED.
Suppose Set B contains 69 elements and the total number elements in either Set A or Set B is 107. If the Sets A and B have 13 elements in common, how many elements are contained in set A?
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