12. After a coordinate paper is folded once, point $A(0,4)$ overlaps with point $B(8,0)$. If point $C(6,8)$ overlaps with point $D(m$, $n)$, then $m+n=$
12.14.8.

The equation of the axis of symmetry can be solved as $y=2x-6$. From $\frac{n+8}{2}=(6+m)-6, \frac{n-8}{m-6}=-\frac{1}{2}$, we get $m=7.6, n=7.2$.