The value that we arrived at is the rate that heat flows through the air across an area the size of the window on either side of the window. Therefore the rate of heat flows out through the window would be 1.8 W.
b) To find out the difference in temperature ΔT between the inner and outer glass surfaces, substitute 1.8 W for H, 0.50 cm for Δx, 1.0 W/m. K for k and (0.6 m)² for A in the equation ΔT = H Δx/kA,
ΔT = H Δx/kA
= (1.8 W) (0.50 cm)/( 1.0 W/m. K)( (0.6 m)²)
= (1.8 W) (0.50 cm×10^-2 m/1 cm )/( 1.0 W/m. K)( (0.6 m)²)
= 0.025 K
From the above observation we conclude that, the difference in temperature ΔT between the inner and outer glass surfaces would be 0.025 K.