68. A uniform cube die, with its six faces marked with the numbers $1,2,3,4,5,6$, is thrown twice. Let the number rolled on the first throw be $a$, and the number rolled on the second throw be $b$. Then, the probability that the system of equations $\left\{\begin{array}{l}a x+b y=3 \\ x+2 y=2\end{array}\right.$ has only positive solutions is $\qquad$.
Answer: $\frac{13}{36}$