\(\dfrac{x+y}{2}=12\Leftrightarrow x+y=24\Rightarrow x=24-y\)

\(\dfrac{y+12}{2}=\dfrac{z}{2}\Leftrightarrow y+12=z\)

=> \(\dfrac{x+z}{2}=\dfrac{24-y+y+12}{2}=\dfrac{36}{2}=18\)

So the mean of x and z is 18