### Solution:

(i) Let us consider, Speed of Ritu in still water = x km/hr and Speed of Stream = y km/hr

Now, speed of Ritu during downstream = x + y km/h and Upstream = x – y km/h

As per the question given,

2(x+y) = 20

x + y = 10……………………….(1)

And, 2(x-y) = 4

x – y = 2………………………(2)

Adding both the equations, we get,

2x = 12 x = 6

Putting the value of x in (1), we get,

y = 4

Therefore, Speed of Ritu in still water = 6 km/hr and Speed of Stream = 4 km/hr