A boat whose speed in still water is 9 kmph, goes 12 km downstream and comes back in 3 hrs. Find the speed of the stream?

Let the speed of the stream = x kmph

From the given data,

  $\frac{12}{9+x}+\frac{12}{9-x}=3 hrs$

$3x2 = 27$

=> x = 3 kmph

Therefore, the speed of the stream = 3 kmph