Discussion

The speed of a boat in still water is 20 km/hr and the rate of current is 5 km/hr. The distance traveled downstream in 12 minutes is:

Answer: C

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

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

\[ b + r = 20 + 5 = 25 \text{ km/h} \]

\[ 12 \text{ min} = \frac{12}{60} = 0.2 \text{ h} \]

\[ \text{distance} = 25 \times 0.2 = 5 \text{ km} \]

Using the still-water speed of 20 km/h instead would give 4 km - the current is helping, so the distance must exceed that.

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