Suppose we have 2 classrooms A and B having n students in each. Our task is to select total of n students out of then and then select a leader from students selected from class A.
Lhs is the cases taken... 0 from A n from B and then multiply by 0 coz can't select a leader from amongst those selected from A
nc1from A nCn-1 from B and then leader in 1 way.... And so on gives LHS
Now we do the same thing in as follows.. Select 1 leader from A first in n ways and then select n-1 from remaining 2n-1 in 2n-1Cn-1ways and that should give u RHS