Ashok can row upstream at 8 kmph and downstream at 12kmph.What is the speed of the stream ?

If the speed downstream is a kmph and the speedup stream is b kmph then
Speed of the stream = 1/2 x (a–b) kmph
Speed downstream a = 12kmph
Speed upstream b = 8 kmph
Speed of the stream = 1/2 x (a–b) = 1/2 x (12–8)= 4/2 = 2 kmph