Explanation
$$\text{Let }E\text{ be the set of students who know English.}$$
$$H\text{ be the set of students who know Hindi.}$$
$$\text{It is given that , }$$
$$n\left(U\right)=100$$
$$n\left(E\right)=20, n\left(\overline{H}\right)=20$$
$$\text{and } n\left(\overline{E\cup H}\right)=10$$
$$\textbf{Step -2: Draw Venn Diagram}$$
$$\textbf{Step -3: Find the number of students who know either English or Hindi.}$$
$$\text{Number of students who know either English or Hindi are:}$$
$$n(E\cup H)=n(U)-n(\overline{E\cup H})$$
$$=100-10$$
$$=90$$
$$\textbf{Step -4: Find the number of students who Hindi.}$$
$$\text{Number of students who know Hindi are:}$$
$$n(H)=n(U)-n(\overline{H})$$
$$=100-20$$
$$=80$$
$$\textbf{Step -5: Find the number of students who know both English or Hindi.}$$
$$\text{Number of students who know both Hindi and English are }n(E\cap H).$$
$$\mathbf{\because n(A\cup B)=n(A)+n(B)-n(A\cap B)}$$
$$\therefore n(E\cup H)=n(E)+n(H)-n(E\cap H)$$
$$\Rightarrow 90=20+80-n(E\cap H)$$
$$\Rightarrow n(E\cap H)=10$$
$$\textbf{Hence, the correct option is B.}$$
If $$Y\cup \left\{ 1,2 \right\} =\left\{ 1,2,3,5,9 \right\} $$, then
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