1 g of ice is mixed with 1 g of steam. After thermal equilibrium is achieved, the temperature of the mixture is:

$$\begin{gathered} 1\times 80+1\times 1\times (\mathrm{\theta }-0)=1\times 540+1\times 1\times (100-\mathrm{\theta }) \\ \Rightarrow \mathrm{\theta }={280}^{\mathrm{o}}\mathrm{C} \\ \mathrm{But} 0^{\mathrm{o}}\mathrm{C}\le \mathrm{\theta }\le {100}^{\mathrm{o}}\mathrm{C} \\ \mathrm{So}, \mathrm{final} \mathrm{temperature} ={100}^{\mathrm{o}}\mathrm{C} \end{gathered}$$