Discussion
A boat goes 6 km in an hour in still water. It takes thrice as much time in covering the same distance against the current. Speed of the current is?
Answer: C
With \( b \) the still-water speed and \( r \) the stream's,
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
Still water gives \( b = 6 \) km/hr. Taking three times as long against the current means going at one third the speed:
\[ b - r = \frac{6}{3} = 2 \text{ km/hr} \]
\[ r = 6 - 2 = 4 \text{ km/hr} \]
The comparison here is with the still-water time, not the downstream time - so \( b - r \) is compared with \( b \), giving \( r = \tfrac{2b}{3} \).
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