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Question
Let A and B be two sets such that : \[n \left( A \right) = 20, n \left( A \cup B \right) = 42 \text{ and } n \left( A \cap B \right) = 4\] \[n \left( B - A \right)\]
Solution
Given:
\[n \left( A \right) = 20, n \left( A \cup B \right) = 42 \text{ and } n \left( A \cap B \right) = 4\]
\[ \text{ We know that sets follow the commutative property } . \]
\[ \therefore n(A \cap B) = n(B \cap A)\]
\[n(B - A) = n(B) - n(B \cap A)\]
\[ \Rightarrow n(B - A) = 26 - 4 = 22\]
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