13. (5 points) Winter has come, and the white rabbit has only stored 180 carrots, while the gray rabbit has only stored 120 cabbages. To ensure they have food to eat during the winter, the gray rabbit traded a dozen cabbages for some carrots from the white rabbit. At this point, the amount of food they stored is equal. Then, one cabbage can be exchanged for $\qquad$ carrots.
【Solution】Solution: Let the little gray rabbit exchange $x$ cabbages for $y$ carrots from the little white rabbit.
$$
\begin{array}{l}
120-x+y=(120+180) \div 2, \\
120-x+y=150,
\end{array}
$$

That is, $y-x=30$,
Also, since $10<x<20$,
Therefore, $\frac{y}{x}=\frac{30}{x}+1$,
To make $\frac{y}{x}$ an integer,
Thus, $x=15$,
$$
y=45 \text {, }
$$

So $y \div x=3$; hence the answer is: 3 .