Discussion
If a boat goes 7 km upstream in 42 minutes and the speed of the stream is 3 kmph, then the speed of the boat in still water is
Answer: B
With \( b \) the still-water speed and \( r \) the stream's,
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ 42 \text{ min} = \frac{42}{60} = 0.7 \text{ h} \]
\[ b - r = \frac{7}{0.7} = 10 \text{ kmph} \]
\[ b = (b - r) + r = 10 + 3 = 13 \text{ kmph} \]
The current is added back once, not twice: subtracting it gave the upstream speed, so restoring it gives the still-water speed. Adding it a second time would give the downstream speed of 16 kmph.
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