In order to ascertain the elevation of the top (Q) of the signal on a hill, observations were made from two instrument stations P and R at a horizontal distance 100 metres apart, the stations P and R being in line with Q. The angles of elevation of Q at P and R were 28^o 42’ and 18^o 6’ respectively. The staff reading upon the bench mark of elevation 287.28 were respectively 3.750 and 2.870 when the instrument was at P and at R, the telescope being horizontal. Determine the elevation of the foot of the signal if the height of the signal above its base is 3 metres.