Let the speed of boat be ‘a’ and speed of stream be ‘b.’

Speed = distance/time

Relative speed of boat going upstream = a – b

Relative speed of boat going downstream = a + b

Given, Ritu can row downstream 20 km in 2 hours, and upstream 4 km in 2 hours

⇒ a + b = 20/2 = 10 ---- (1) and a – b = 4/2 = 2 ------ (2)

Adding eq1 and eq2

⇒ 2a = 12

⇒ a = 6 km/hr

Also, b = a – 2 = 4 km/hr