Discussion

A boat is rowed downstream at 15.5km/hr and upstream at 8.5 km/hr. The speed of the stream is?

Answer: A

With \( b \) the still-water speed and \( r \) the stream's,

\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]

\[ b = \frac{(b+r) + (b-r)}{2}, \qquad r = \frac{(b+r) - (b-r)}{2} \]

\[ r = \frac{15.5 - 8.5}{2} = \frac{7}{2} = 3.5 \text{ km/hr} \]

The boat does 12 km/hr in still water. The 7 km/hr option is the raw difference, which is twice the current.

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