I4.1 Given that the means of $x$ and $y, y$ and $z, z$ and $x$ are respectively 5,9 , 10. If $a$ is the mean of $x$, $y, z$, find the value of $a$.
$\begin{array}{l}\frac{x+y}{2}=5 \ldots(1) ; \quad \frac{y+z}{2}=9 \ldots(2) ; \frac{z+x}{2}=10 \ldots \\ (1)+(2)+(3): x+y+z=24 \\ \Rightarrow a=8\end{array}$