Question

If a man rows @ 11.5 km h in still water and his speed upstream is 7 km h what is his speed downstream

Answer 1

Speed of boat in still water = 11.5 kmph
Upstream speed = 7 kmph

Upstream speed = speed of boat in still water - speed of stream
7 = 11.5 - speed of stream
speed of stream = 11.5 - 7 =4.5 kmph 

Downstream speed = speed of boat in still water + speed of stream
Down stream speed = 11.5 + 4.5 = 16 kmph
Hope this helps