A this usual rowing rate Mohit can travel 12 miles downstream in a certain river in 6 hours less tha...........

Explanation:

Speed of the stream = ½ (a-b) km/hr
Explanation:
Let the speed in still water be x m/hr
Speed of stream be y m/hr
Then, speed upstream = x-y m/hr and
Speed downstream = x+y m/hr
12/x-y – 12 / x+y = 6 so,6 (x^2 – y^2) = 24 y
x^2 – y^2 = 4y
x^2 = y^2 + 4y…………..1
also
12/ 2x-y – 12/2x +y = 1 4x^2 – y^2 = 24y
x^2 = [24y + y^2] / 4 ……………….2
16y + 4y^2 = 24y + y2 [put X^2 value from 1] 3y^2 = 8 y so, y = 8/3
speed of the current = 8/3 m/hr = 2 (2/3) m/h