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?
Set A or B refers to the union of A and B, in other words how many element there are in TOTAL. We are told Set B has 69 elements, but that 13 of them overlap with Set A. This means Set B has 69-13=56 unique elements, and all others are therefore part of set A. If there are 107 elements in total, Set A has 107-56=51 elements.