Discussion

A boat covers 36 km downstream in 3 hours and the same distance upstream in 6 hours. What is the speed of the boat in still water ?

Answer: A

Correct answer: 9 km/hr

\[\text{Downstream speed} = \frac{36}{3} = 12 \text{ km/hr}, \qquad \text{Upstream speed} = \frac{36}{6} = 6 \text{ km/hr}\]

The still-water speed is the average of the two, since the stream adds to one and subtracts from the other:

\[b = \frac{12 + 6}{2} = 9 \text{ km/hr}\]

The same pair gives the stream speed as \(\tfrac{12-6}{2} = 3\) km/hr.

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