If 4 moles of an ideal monoatomic gas at temperature $400 \;K$ is mixed with 2 moles of another ideal monoatomic gas at temperature $700\;K$ the temperature of the mixture is :

Question

$$\begin {array} {1 1} (a)\;550^{\circ}C & \quad (b)\;500^{\circ}C \\ (c)\;550\;K & \quad (d)\;500\;K \end {array}$$