Discussion
A boat can move upstream at 25 kmph and downstream at 35 kmph, then the speed of the current is?
Answer: A
With \( b \) the still-water speed and \( r \) the stream's,
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b = \frac{(b+r) + (b-r)}{2}, \qquad r = \frac{(b+r) - (b-r)}{2} \]
\[ r = \frac{35 - 25}{2} = 5 \text{ kmph} \]
The current's speed is half the difference, not the difference itself: it helps by \( r \) one way and hinders by \( r \) the other, so the gap between the two speeds is \( 2r \). That is why 10 kmph is on the list.
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