[khanna_sumit]

Originally Posted by suprovo2000

I do not understand ....3!/2! -3!/3!....Please explain this properly.......

Suprovo.

this comes from the formula for derangement....if u have n letters and u want to put these letters in n addressed envelops such that none of the letters goes to its correct envelop then that can be done in n!*(1/2! - 1/3! + 1/4! - 1/5! + 1/6!.....1/n!)