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#### karush

##### Well-known member

- Jan 31, 2012

- 2,739

(b) $A'=100-30=70$ and since $n(A\cup B) = 65$ and $A+B=80$

then $n(A\cap B)= 15$ so $n(B\cap A')=70+15=85$

(c) $\displaystyle\frac{n(B\cap A')}{n(U)}=\frac{85}{100}=\frac{17}{20}$

or ????