A spherical tank is filled with ice slurry, where its inner surface is at 0°C. The tank has an inner diameter of 9 m and its wall thickness is 20 mm. The tank wall is made of a material with a thermal conductivity given as k(T) = k₀ (1 + βT), where k₀ = 0.33 W/m ∙ K, β = 0.0025 K⁻¹, and T is in K. The tank is in a surrounding temperature of 35°C and a convection heat transfer coefficient of 70 W/m² ∙ K. Solar radiation is incident on the tank’s outer surface at a rate of 150 W/m², where the emissivity and solar absorptivity of the outer surface are 0.75. Determine the outer surface temperature of the tank.