1*(1-(1)) * (1-(2)) * (1 -( 3)) * ... * (1- (k-1))

how is above statement is

Let n be the distinct values in the first four characters Suppose there are k students in the class where k ? N and k ? n

n = 26 x 12 x 10 = 3120 (3) Suppose there are & students in the class where k E N and k n, obviously there will be collision.) Po = 1 x (1 - - -) x (1 . k - 1 X . . . x 1- k(1-k) (4) 2n