Question

On the occasion of New Year, each student of a class sends greeting cards to the others. If there are 21 students in the class, what is the total number of greeting cards exchanged by the students?
A) 380
B) 420
C) 441
D) 400

Answer 1

If every person gives cards to every other person then the condition will be n(n-1) where n is equals to number of students.
21(20)=420