Zeroth law of thermodynamics
Zeroth law of thermodynamics states that if two systems A and B are in thermal equilibrium with a third system C separately (that is A and C are in thermal equilibrium; B and C are in thermal equilibrium) then A and B are in thermal equilibrium themselves.
Let us say Ta, Tb and Tc are the temperatures of A, B and C respectively.
A and C are in thermal equilibrium. Ta = Tc
B and C are in thermal equilibrium. Tb = Tc