How fast is the stream?

Assume $v_B,v_R$ to be the speeds of the boat and the river respectively.

Let the boat covers $x$ distance in $1$ hr. with a speed $v_B+v_R$ since downstream ,therefore $x=v_B+v_R$

Now when it returns at the edge of $6$ kms before the point A he revisits the raft. Int his upstream the distance he travelled is $v_B+v_R-6$ with a speed $v_B-v_R$

So total time consumed, $\displaystyle T = 1+\dfrac{v_B+v_R-6}{v_B-v_R}$

Again this is the time in which the raft traversed $6$ km with the flow of the river so we must have $T=\dfrac{6}{v_R}$

$\displaystyle \begin{aligned} 1+\dfrac{v_B+v_R-6}{v_B-v_R}=\dfrac{6}{v_R}\\ v_B-v_R+v_B+v_R-6=\dfrac{6}{v_R}\left(v_B-v_R \right) \\ v_R(2v_B-6)=6(v_B-v_R) \\ v_B v_R=3v_B \\ v_R=\boxed{3}\end{aligned}$

As the boat will travel at a constant speed wrt to river therefore the relative speed is same for both upstream and downstream. Therefore it takes same time to reach the raft (60 Min or 1 Hour) as raft is floating along river. The raft has traveled a distance of 6 km wrt ground, therefore the speed is 3 kmph.