Question Number 169432 by mathlove last updated on 30/Apr/22
$${n}\left({A}\right)=\mathrm{7} \\ $$$${n}\left({A}\cap{B}\right)=\mathrm{4} \\ $$$$\mathrm{3}{n}\left({A}−{B}\right)={n}\left({B}\right) \\ $$$${n}\left({B}−{A}\right)=? \\ $$
Answered by blackmamba last updated on 30/Apr/22
$$\:{n}\left({A}−{B}\right)=\:{n}\left({A}\right)−{n}\left({A}\cap{B}\right) \\ $$$$\Rightarrow\mathrm{3}\left(\mathrm{7}−\mathrm{4}\right)={n}\left({B}\right)\:;\:{n}\left({B}\right)=\mathrm{9} \\ $$$${n}\left({B}−{A}\right)={n}\left({B}\right)−{n}\left({A}\cap{B}\right)=\mathrm{9}−\mathrm{4}=\mathrm{5} \\ $$