$0.050\text{kg}$ of ice at $-10^{\circ }\text{C}$ is added to $0.30\text{kg}$ of water at $40^{\circ }\text{C}$ in a well-insulated container. What is the final temperature, in ${}^{\circ }\text{C}$? (Specific heat capacity of ice $2100{\text{J kg}}^{-1}{\text{K}}^{-1}$, of water $4180{\text{J kg}}^{-1}{\text{K}}^{-1}$; specific latent heat of fusion $3.34\times 10^5{\text{J kg}}^{-1}$. Assume no heat is exchanged with the container.)