7. A raft departed from point $A$ to point $B$ downstream the river. After 2.4 hours, a motorboat (with its own speed of 20 km/h) set off in pursuit of the raft. The motorboat caught up with the raft and immediately turned back to point $A$. After 3.6 hours, the motorboat arrived at point $A$, while the raft reached point $B$. Determine the speed of the river current.
7. Let the speed of the river current be $x$ km/h, $t$-the time elapsed from the moment the boat left until the moment of the meeting. Then, by the condition: 1) the distance traveled by the raft until the meeting, 2) the distance traveled by the motorboat until the meeting, 3) the distance traveled by the motorboat after the meeting, are equal to each other. From this we get:

$(2.4+t) x=t(20+x)=(3.6-2.4-t)(20-x)$. From this we find: $t=\frac{2.4}{20} x \Rightarrow x=4$. Answer: 4 km/h.