jmc

We can simplify as follows: $$\begin{array}{rlr}13(3x-2)& \equiv 26& (\mathrm{mod}8)\\ 3x-2& \equiv 2& (\mathrm{mod}8)\\ 3x& \equiv 4& (\mathrm{mod}8)\\ 9x& \equiv 4\cdot 3& (\mathrm{mod}8)\\ x& \equiv 12& (\mathrm{mod}8)\\ x& \equiv 4& (\mathrm{mod}8)\end{array}$$So $x=4+8n$ is a solution for all $n$ and all solutions are of this form. The solutions in the range $0<x\le 20$ are $4,12,20$, so their sum is $4+12+20=\boxed{36}$.