In a group of students 100 students know Hindi, 50 know English and 25 know both.

Let U be the set of all students in the group.

Let E be the set of all students who know English.

Let H be the set of all students who know Hindi.

$\therefore H \cup E=U$

Accordingly, $n(\mathrm{H})=100$ and $n(\mathrm{E})=50$

$n(\mathrm{H} \cap \mathrm{E})=25$

$n(\mathrm{U})=n(\mathrm{H})+n(\mathrm{E})-n(\mathrm{H} \cap \mathrm{E})$

= 100 + 50 – 25

= 125

Hence, there are 125 students in the group.