imc

The average of all valid numbers is simply the expected value of a randomly chosen valid number. In other words, it is $E[\text{ten thousands digit}]\cdot 10^4+E[\text{thousands digit}]\cdot 10^3+\cdots +E[\text{ones digit}]\cdot 1$. However, every digit is equally likely to be chosen for every place, so we can simplify this to $\frac{1+3+5+7+8}{5}\cdot (10^4+10^3+10^2+10^1+1)$ $=4.8\cdot 11111=\boxed{53332.8}$.