Discussion
A boat covers a distance of 30 km in 2 ½ hours running downstream while returning it covers the same distance in 3 ¾ hours. What is the speed of the boat in still water?
Answer: D
With \( b \) the still-water speed and \( r \) the stream's,
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b + r = \frac{30}{2.5} = 12 \text{ km/hr}, \qquad b - r = \frac{30}{3.75} = 8 \text{ km/hr} \]
\[ b = \frac{12 + 8}{2} = 10 \text{ km/hr} \]
10 km/hr is absent from the options, so the answer is none of these. The 12 and 8 on the list are the downstream and upstream speeds - intermediate values, not the answer.
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