Question

If $M$ and $N$ are two sets such that $n\left(M\right)=17$, $n(M\cup N)=38$ and $n\left(N\right)=23$, find $n(M-N)$.

• A. 15
• B. 12
• C. 8
• D. 7

Answer 1

The correct option is A 15

$n(M\cup N)$ = $n\left(M\right)$ + $n\left(N\right)$- $n(M\cap N)$

38 = 17 + 23 - $n(M\cap N)$

$n(M\cap N)$ = 2

Now, $n(M-N)$= $n\left(M\right)$- $n(M\cap N)$

$n(M-N)$ = 17 - 2 = 15