The temperature at a point (x,y,z) of a solid E bounded by the coordinate planes and the plane $\displaystyle {2}\cdot{x}+{y}+{z}={1}$ is $\displaystyle {T}{\left({x},{y},{z}\right)}={\left({x}{y}+{8}{z}+{20}\right)}$ degrees Celcius. Find the average temperature over the solid.  (Answer to 4 decimal places).