13. Use the digits $1,2,3,4,5,6,7,9$ to form 4 two-digit prime numbers (each digit must be used exactly once). How many different combinations of these 4 prime numbers are possible?
【Answer】 4
【Solution】 It is easy to know that $2, 4, 5, 6$ can only be used as the tens digit. Let these four two-digit prime numbers be $\overline{2 a}, \overline{4 b}, \overline{5 c}, \overline{6 d}$. The remaining four digits are $1, 3, 7, 9$. A simple analysis shows that $a \neq 1,7, b \neq 9, c \neq 1,7, d \neq 3,9$. Since $\left\{\begin{array}{l}a \neq 1,7 \\ c \neq 1,7\end{array} \Rightarrow a, c\right.$ can only take $3, 9$, and the remaining $1, 7$ must be assigned to $b, d$. There are a total of $2 \times 2=4$ possibilities (the first 2 represents $\left\{\begin{array}{l}a=3 \\ c=9\end{array}\right.$ or $\left\{\begin{array}{l}a=9 \\ c=3\end{array}\right.$, the second 2 represents $\left\{\begin{array}{l}b=1 \\ d=7\end{array}\right.$ or $\left\{\begin{array}{l}b=7 \\ d=1\end{array}\right)$