Help Physical sound question!?

Area of partition = 4 x 2.5 m^2 = 10 m^2
Area of window = 1.2 x 0.8 m^2 = 0.96 m^2
Area of door = 2 x 0.75 m^2 = 1.5 m^2
Area of brick wall = (10 - 0.96 - 1.5) m^2 = 7.54 m^2

Let Tb be the transmission coefficient of the brick wall.
53 = 10.log(1/Tb)
hence, Tb = 10^(-53/10) = 5.012 x 10^-6

Let Tw be the transmission coefficient of the window
25 = 10.log(1/Tw)
hence, Tw = 10^(-25/10) = 3.162 x 10^-3

Let Td be the transmission coefficient of the door
20 = 10.log(1/Td)
hence, Td = 10^(-20/10) = 0.01

Let Tav be the average transmission coefficient of the complete partition, we have,
10(Tav) = 7.54(Tb) + 0.96(Tw) + 1.5(Td)
hence, Tav = [7.54 x (5.012 x 10^-6) + 0.96 x (3.162 x 10^-3) + 1.5 x 0.01]/10
i.e. Tav = 1.807 x 10^-3

The overall sound reduction index
= 10.log(1/1.807 x 10^-3) dB
= 27.43 dB