Rozly can row downstream 20km in 2 hours, and the upstream 4km in 2 hours. What will be the speed of rowing in still water?

To find the speed of rowing in still water, we can use the concept of relative velocity.

Let's assume the speed of the current is C km/h and the speed of rowing in still water is R km/h.

When Rozly rows downstream, her effective speed is the sum of her rowing speed and the speed of the current. So, her effective speed downstream is R + C km/h.

Given that she can row downstream 20 km in 2 hours, we can set up the equation:

20 km = (R + C) km/h * 2 hours

Simplifying the equation, we get:

10 km/h = R + C

Similarly, when Rozly rows upstream, her effective speed is the difference between her rowing speed and the speed of the current. So, her effective speed upstream is R - C km/h.

Given that she can row upstream 4 km in 2 hours, we can set up the equation:

4 km = (R - C) km/h * 2 hours

Simplifying the equation, we get:

2 km/h = R - C

Now, we have a system of equations:

10 km/h = R + C 2 km/h = R - C

We can solve this system of equations to find the values of R and C.

Adding the two equations, we get:

12 km/h = 2R

Dividing both sides by 2, we find:

R = 6 km/h

Now that we have the value of R, we can substitute it back into one of the original equations to find the value of C.

Using the equation 10 km/h = R + C, we get:

10 km/h = 6 km/h + C

Subtracting 6 km/h from both sides, we find:

C = 4 km/h

Therefore, the speed of rowing in still water is 6 km/h.