Discussion
Vijay rows a boat 80 km along the stream in 1 hour 20 minutes. Given that his rowing speed in still water is 45 km/hr, in how much time will he cover the same distance against the stream?
Answer: C
Write \( b \) for the boat's speed in still water and \( r \) for the stream's. Then
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ 1 \text{ h } 20 \text{ min} = \frac{4}{3} \text{ h} \]
\[ b + r = \frac{80}{4/3} = 60 \text{ km/h} \]
\[ r = 60 - 45 = 15 \text{ km/h} \quad\Longrightarrow\quad b - r = 45 - 15 = 30 \text{ km/h} \]
\[ t = \frac{80}{30} = \frac{8}{3} \text{ h} = 2 \text{ h } 40 \text{ min} \]
The upstream speed is half the downstream speed here, so the return leg takes exactly twice as long.
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