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  4. For two sets A and B , if n(A)=20,n(B)=18 and n(A ∩ B)=5 , then n(A - B) is equal to

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Q. For two sets $A$ and $B$ , if $n\left(A\right)=20,n\left(B\right)=18$ and $n\left(A \cap B\right)=5$ , then $n\left(A - B\right)$ is equal to

NTA AbhyasNTA Abhyas 2020

Solution:

$n\left(A - B\right)=n\left(A \cup B\right)-n\left(B\right)=n\left(A\right)-n\left(A \cap B\right)$
$=20-5$
$=15$