13. The military training base purchases apples to comfort the trainees. It is known that the total number of apples, when represented in octal (base 8), is $\overline{a b c}$, and when represented in septenary (base 7), it is $\overline{c b a}$. Therefore, the total number of apples, when represented in decimal (base 10), is $\qquad$
13.220.
$$
\begin{array}{l}
\text { Given } 1 \leqslant a, b, c \leqslant 6, \\
a \times 8^{2}+b \times 8+c \\
=c \times 7^{2}+b \times 7+a, \\
63 a+b-48 c=0, \\
b=3(16 c-21 a),
\end{array}
$$

Therefore, $b=0,3,6$.
Upon verification, $b=3$ meets the condition. Hence, $b=3, c=4, a=3$. Thus, $3 \times 8^{2}+3 \times 8+4=220$.