Motorboat and Raft

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Problem:
A motorboat going downstream overcame a raft at a point $A$; $T = 60$ minutes later it turned back and after some time passed the raft at a distance $\tau = 6.0 \text{km}$ from the point A. Find the flow velocity assuming the duty of the engine to be constant. 



Solution:

Think of the situation only after point $A$, as it doesn't matter what happened before it.

Note that the motorboat moves away from $A$ for $1$ hour before it turns back, so it will take exactly the same time to get back to it. Thus it will take a total of $2$ hours for this entire process. In the meantime of this, $A$ has moved downstream $6 \text{km}$ with respect to the shore, and hence the flow velocity is $\frac{6}{2}=3$. 

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