The coast of a river

Question

On the coast of a river, there are four docks : A, B, C, D (in that order). It takes a ferry two hours to get from point B to point A, one hour from point B to point D and three hours from point C to point A. In what direction does the river flow - from A to D or from D to A? Explain the solution.

Answer 1

With obvious notation:

$$1) \qquad(C\to B)=(C\to A)-(B\to A)=3-2=1$$

$$2)\qquad \left((B\to D)=1 \quad \mbox{and} \quad D\ne C\right) \Rightarrow (B\to C)<1$$

so: $(C\to B)>(B\to C)$ and the river flows as $A\to B\to C\to D$.