It is known that it takes four hours for a ship to sail 48 kilometers downstream and six hours to sail 48 kilometers upstream. Now the ship is from the upstream city A to the downstream city B. It is known that the waterways of the two cities are 72 kilometers long. During the voyage, a passenger threw a board from the window and asked how many kilometers it was from B city when the ship arrived.

Analysis:

The downlink speed is 48÷4= 12 (km), and the downlink speed is 48÷6=8 (km).

Because the downstream speed is greater than the ship speed, and the upstream speed is the ship speed minus the water speed.

Therefore, the difference between the downstream velocity and the upstream velocity is "two-water velocity", so the outlet velocity can be obtained by the following formula:

(12-8)÷2=2 (km).

At present, the ship is from the upstream city A to the downstream city B, so it runs smoothly. The time from A to B is 72÷ 12=6 (hours).

The time from the starting point to the end point is the same as that of a ship. The board floats with the water, so the speed of driving is the speed of water. The distance of the board within 6 hours can be calculated as follows:

6×2= 12 (km); The distance from the ship to B is still very short: 72- 12=60 (km).

Answer:

Solution: downstream velocity 48÷4= 12 (km),

The upstream speed is 48÷6=8 (km),

The velocity of water is: (12-8)÷2=2 (km),

The time from a to b is: 72÷ 12=6 (hours),

The 6-hour plank road distance is 6×2= 12 (km).

The distance from the ship to B is still very short: 72- 12=60 (km).

Answer: When the ship arrived at Port B, the wooden block was 60 meters away from Port B. 。

Comments: This topic uses the relationship between (downstream speed-upstream speed) ÷2= water flow speed.