Heat required to melt the ice = m * L = 50 g * 80 cal/g = 4000 cal. Heat available from the water cooling from 80°C to 0°C = m * s * ΔT = 50 g * 1 cal/g°C * 80°C = 4000 cal. Since the heat available is exactly enough to melt all the ice, the final temperature of the mixture will be 0°C.