In a group of students, 100 students know Hindi, 50 know English and 25 know both. Each of the students knows either Hindi or English. How many students are there in the group?

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
Given n(H)=100 and n(E)=50
n(H∩E)=25
n(U) = n(H) + n(E) - n(H∩E) = 100 + 50 - 25 = 125
Hence, there are 125 students in the group.