There are 5 doors in a room, 2 on one side and 3 on the other side. A man wishes to enter the room from the side with two doors and get out from the other side. In how many ways can this be done?

                                    
Number of ways in which he can enter = 2
                         m = 2
Number of ways in which he can leave the room = 3
                          n = 3
By fundamental principle of counting, the number of ways in which he can enter the room from one side and get out from the outer side = m x n = 2 x 3 = 6