Question

Q.6. If n(A) = 3, n(B) = 6 and $A\subseteq B$, then then number of elements in A$\cup$B are
(1) 6
(2) 7
(3) 5
(4) 3

Answer 1

$$\begin{gathered} SinceA\underline{\subset }B \\ Therefore \\ A\cap B=A \\ \Rightarrow n\left(A\cap B\right)=n\left(A\right) \\ n\left(A\cup B\right)=n\left(A\right)+n\left(B\right)-n\left(A\cap B\right) \\ n\left(A\cup B\right)=n\left(A\right)+n\left(B\right)-n\left(A\right) \\ n\left(A\cup B\right)=n\left(B\right)=6 \end{gathered}$$