The temperature of equal masses of three different liquids A, B and C are ${12}^{\mathrm{ o}}\mathrm{C}, {19}^{\mathrm{ o}}\mathrm{C}$ and ${28}^{\mathrm{ o}}\mathrm{C}$ respectively. The temperature when A and B are mixed is ${16}^{\mathrm{ o}}\mathrm{C}$ and when B and C are mixed is ${23}^{\mathrm{ o}}\mathrm{C}$. The temperature when A and C are mixed is