In a group of $500$ students, there are $475$ students who can speak Hindi and $200$ can speak Bengali. What is the number of students who can speak Hindi only ? 

Total number of students $= 500$

Let H be the set showing number of students who can speak Hindi $= 475$

B be the set showing number of students who can speak Bengali $= 200$  

So, $n\left(H\right)=475\text{ and }n\left(B\right)=200$ and given that $\mathrm{n}(\mathrm{B}\cup \mathrm{H})=500$

$\begin{array}{r}\mathrm{n}(\mathrm{B}\cup \mathrm{H})=\mathrm{n}\left(\mathrm{B}\right)+\mathrm{n}\left(\mathrm{H}\right)-\mathrm{n}(\mathrm{B}\cap \mathrm{H})\\ \Rightarrow 500=200+475-\mathrm{n}(\mathrm{B}\cap \mathrm{H})\end{array}$

so, $n(B\cap H)=175$

Hence, persons who speak Hindi only $=n\left(H\right)-n(B\cap H)$

$=475-175=300$