4. A truck is traveling at a constant speed on the highway. Initially, the number on the milestone is $\overline{A B}$. After 1 hour, the number on the milestone is $\overline{B A}$. After another hour, the number on the milestone is $\overline{A 0 B}$. Find the numbers seen each time and the speed of the truck.
$$
\begin{array}{c}
\text { (Hint: }(10 B+A)-(10 A+B)=(100 A+B) \\
-(10 B+A), A=1, B=6 . \overline{A B}=16, \overline{B A}=61, \overline{A 0 B}=
\end{array}
$$
106. Speed is 45 kilometers/hour.