Here we must write and solve a system of equations to find the speed of the boat and the speed of the stream. We will find that the boat's speed is 23km/h and the river's speed is 4km/h.
First, remember the relation:
Distance = Speed*Time.
Now let's define the variables we will be using:
- B = boat's speed
- R = river's speed.
When the boat travels downstream, the total speed of the boat is the speed of the boat plus the speed of the river, and we know that in that case it travels 54km in 2 hours, then:
54km = (B + R)*2h
When the boat travels upstream, we must subtract the speed of the river. In that case, we know that the boat travels 57km in 3 hours, then we have:
57km = (B - R)*3h
Then our system of equations is:
54km = (B + R)*2h
57km = (B - R)*3h
To solve this, first, we need to isolate one of the variables in one of the equations, let's isolate B in the second one:
57km = (B - R)*3h
57km/3h + R = B
19 km/h + R = B
Now we can replace that in the other equation:
54km = (B + R)*2h
54km/2h = B + R
27 km/h = B + R = (19 km/h + R) + R
Now we can solve this for R:
27 km/h = 19km/h + 2*R
27 km/h - 19km/h = 2*R
8 km/h = 2*R
(8km/h)/2 = 4km/h = R
Now that we know the value of R, we can use:
B = 19 km/h + R = 19 km/h + 4km/h = 23 km/h
So the boat's speed is 23km/h and the river's speed is 4km/h.
If you want to learn more, you can read:
brainly.com/question/12895249
added with 
hours should be equal or greater than 
(helpful insight from the keyword "at least"). Therefore, it's inequality would look like:
(>= is used instead of ≥ for constraints in formatting)