4. A person takes a boat from place $A$ downstream to place $B$, and then upstream to place $C$, spending a total of 4 hours on the boat. It is known that the speed of the boat in still water is 7.5 kilometers per hour, and the speed of the current is 2.5 kilometers per hour. If the distance between $A$ and $C$ is 10 kilometers, then the distance between $A$ and $B$ is $\qquad$ kilometers.
(Second Junior High School Mathematics Open Contest, "Learning Newspaper")
$\begin{array}{l}4.20 \\ \text { or } \frac{20}{3}\end{array}$