A watermelon initially at 35°C is to be cooled by dropping it into a lake at 15°C. After 4 h and 40 min of cooling, the center temperature of watermelon is measured to be 20°C. Treating the watermelon as a 20-cm-diameter sphere and using the properties k = 0.618 W/m · K, α = 0.15 × 10^-6 m²/s, ρ = 995 kg/m³, and cp = 4.18 kJ/kg · K, determine the average heat transfer coefficient and the surface temperature of watermelon at the end of the cooling period. Solve this problem using analytical one-term approximation method (not the Heisler charts).