A8. A motorboat moves relative to the water at a constant speed of 25 kilometers per hour. It sails from Arnhem to Zwolle with the constant current. At a certain moment, it has covered $42\%$ of the distance. From that point, it takes the same amount of time to continue sailing to Zwolle as it does to sail back to Arnhem.

What is the speed of the current in kilometers per hour?
A) 3
B) 4
C) $\frac{9}{2}$
D) 5
E) 6

## B-questions
A8. B) 4 From the mentioned point, we can travel $42\%$ of the distance upstream and $58\%$ of the distance downstream in the same time. This means that the boat goes $\frac{58}{42}$ times faster with the current than against it. If we denote the current speed as $v$, we then find that $\frac{25+v}{25-v}=\frac{58}{42}$. This gives $58 \cdot(25-v)=42 \cdot(25+v)$, or $1450-58 v=1050+42 v$. We find that $400=100 v$, or $v=4$.