12. (5 points) Two people, A and B, start from points $A$ and $B$, which are 10 kilometers apart, and walk towards each other. If they start at the same time, they will meet 1 kilometer away from the midpoint of $A$ and $B$. If A starts 5 minutes later, they will meet at the midpoint of $A$ and $B$. At this time, A has walked $\qquad$ minutes.
【Answer】Solution: If A starts 5 minutes late, they will meet at the midpoint of $A$ and $B$. Let's assume A has walked for $x$ minutes, then:
$$
\begin{aligned}
\frac{5}{\mathrm{x}}: \frac{5}{\mathrm{x}+5} & =3: 2 \\
(x+5): \quad x & =3: 2 \\
3 x & =2 x+10 \\
x & =10
\end{aligned}
$$

Answer: A has walked for 10 minutes.
Therefore, the answer is: 10.