2. When they meet for the first time, the distance between Party A and Party B is the same and the time is the same, which means that the speed of Party A is the same, that is, the downstream speed of Party A = the countercurrent speed of Party B. Combining with the speed relationship, we can know that the hydrostatic pressure speed A+water speed B- water speed, so we can know that the hydrostatic pressure speed A +2 water speed is different by two water speeds, that is, the hydrostatic pressure speed A+2 water speed = still water speed. ..
3. From the first meeting to the second meeting, the two ships made two complete trips, of which (after the first meeting, the two ships arrived at their respective destinations) B's upstream trip was half and A's downstream trip was half. In this diversion, the time taken by A and B is the same, that is, they arrive at their respective destinations at the same time, as before meeting for the first time. That is, half of the whole journey of both parties.
Next, the countercurrent of A and the downstream of B constitute another whole journey, but the time spent by the two ships in this whole journey is different, because their speeds are different. For A, the countercurrent velocity = the still water velocity of A-the current velocity, while the downstream velocity of B = the still water velocity of B+the current velocity.
5. The difference between Party B's downstream speed and Party A's upstream speed = Party B's still water speed+current speed-(Party A's still water speed-current speed). Combined with the analysis of the relationship obtained at the first meeting, it can be obtained by substitution: speed difference = Party A's still water speed+twice water speed+water speed-(Party A's still water speed-water speed).
6. So when we met for the second time, A walked less than B 1 km, and it was on this branch road that we walked less. In the whole process of this encounter, Party A and Party B met at the same time (starting from the original destination and meeting on the way), so 1 km is the speed difference between Party B's downlink speed and Party A's uplink speed *. The speed difference can be expressed, but if you don't know the time required, there is no way to calculate the speed of the water flow.
7. From the first meeting to the second meeting, it is divided into two parts. The first part is that after the first meeting, Party A and Party B continue to sail to their respective destinations. The situation was the same as before they first met, so they arrived at the same time. The second part is to return to the second meeting at the same time after reaching their respective destinations. So the time spent in the first part is equivalent to the sum of the whole journey/the speed of two ships, and the time spent in the second part is also the sum of the whole journey/the speed of two ships. Regardless of the upstream, the sum of the speeds of the two ships is still water speed A+ still water speed B (for example, upstream speed A+ downstream speed B = still water speed A- water speed B+ water speed, downstream speed A+ downstream speed B = still water speed A+ water speed B- water speed), so that we can know the two parts that will be used for the first time.
8. According to the distance difference of 1km and the speed difference of step 6, four water flow speeds and the time taken for 45 minutes (3/4 hours), the water flow speed can be calculated.
Water velocity: 1 km /45 minutes /4 = 1 km /(3/4 hours) /4 = 1/3 km/h.
Comprehensive formula: 1 hour 30 minutes =90 minutes, 90 minutes /2=45 minutes =3/4 hours, 1 km /(3/4 hours) /4= 1/3 km/hour.
Although the process is complicated, the thinking is clear.
I hope I can help you.