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A boat running downstream covers a distance of 16 km in 2 hours while for covering the same distance upstream, it takes 4 hours. What is the speed of the boat in still water?

  • A.4 km/hr
  • B.6 km/hr
  • C.8 km/hr
  • D.Data inadequate

Answer: B

Write \( b \) for the boat's speed in still water and \( r \) for the stream's. Then

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

\[ b + r = \frac{16}{2} = 8 \text{ km/h}, \qquad b - r = \frac{16}{4} = 4 \text{ km/h} \]

\[ b = \frac{8 + 4}{2} = 6 \text{ km/h} \]

Two times over a known distance always determine both speeds, so "data inadequate" is never right in this configuration.

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