7.4. The steamship "Raritet" after leaving the city moves at a constant speed for three hours, then drifts for an hour, moving with the current, then moves for three hours at the same speed, and so on. If the steamship starts its journey from city A and heads to city B, it takes 10 hours. If it starts from city B and heads to city A, it takes 15 hours. How long would it take to travel from city A to city B on a raft?
Answer: 60 hours.

Solution: Let the speed of the steamboat be $U$, and the speed of the river be $V$. When the steamboat travels from B to A, it arrives at the destination just before the fourth engine stop, meaning it travels 12 hours at a speed of $U-V$ towards A, and then 3 hours back at a speed of $V$ towards B. Therefore, the distance between the cities is $S=12(U-V)-3 V=12 U-15 V(1)$.

When the steamboat travels from A to B, it manages to break down twice and then continue for another two hours on its own. So, it travels 8 hours at a speed of $U+V$ towards B and 2 hours at a speed of $V$ with the current towards B. Thus, the distance is $S=8(U+V)+2 V=8 U+10 V(2)$. Equating the two, we get $12 U-15 V=8 U+10 V$ or $4 U=25 V$ or $(U / V)=(25 / 4)$. The time it will take to travel by raft is $S / V=12(U / V)-15(V / V)=12 *(25 / 4)$ - 15 $=60$ hours.

Criteria: Only the answer - 0 points. Correctly formulated equation (1) or (2) - 1 point (these points are cumulative).