Let H = set of students, who know Hindi 

E = set of students, who know English 

Then n(H) = 100; n(E) = 50 and n(H∩E) = 25. 

∴ n(H∪E) = n(H) + n(E)-n(H∩E) 

= 100 + 50 – 25 = 125 

Hence, 125 students are there in the group.

Here, Number of students who know Hindi,`n(A) = 100`
Number of students who know English,`n(A) = 50`
Number of students who know both languages,`n(AnnB) = 25`
As each of the students knows either Hindi or English, total number of students will be `n(AuuB)`.
We know,`n(AuuB) = n(A)+n(B)-n(AnnB) = 100+50-25 = 125
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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.
∴ H ∪ E = U
Accordingly, n(H) = 100 and n(E) = 50
n( H U E ) = n(H) + n(E) – n(H ∩ E)
= 100 + 50 – 25
= 125
Hence, there are 125 students in the group.