why is the enthalpy of sublimation is equal to the sum of enthalpy of fusion and enthalpy of vapourization?

Sublimation is the process of converting solid directly into gas.
The whole process can be divided into two parts
1. Melting of solid into liquid (Fusion)
2. Vaporization of liquid o the gas (Vaporization)

Now, according to the Hess’s Law, if a chemical change takes place by several different routes, the overall enthalpy change is the same, regardless of the route by which the chemical change occurs.
Thus enthalpy of sublimation = enthalpy of fusion +enthalpy of vapourizatio