This is incorrect the probability of Mrs. Brown is

hence over all probability =

The probability is with respect to the seat selection after two are occupied.

This argument is incorrect. The correct argument goes like this you have picked

suitable places for Mrs. Brown and

suitable places for Mrs.Lin. Now your job is to place one student in front of Mrs.Lin; and the choices you have are

and the remaining people can pick the seats in

ways. And as you have mentioned total possible arrangements are

. Probability of the event is

Kalyan.