Discussion
The speed of a swimmer in still water is 20 kmph and the rate of flow of the river is 4 kmph. Moving with the stream the swimmer went 120 km. What distance will the swimmer cover during the same time going against the stream?
Answer: B
Against the stream the swimmer covers 80 km.
Downstream speed is 20 + 4 = 24 km/h and upstream speed is 20 - 4 = 16 km/h:
\[ \begin{aligned} t &= 120/24 = 5\ \text{h} \\ d &= 16 \times 5 = 80\ \text{km} \end{aligned} \]
Equivalently, in equal times distances are in the ratio of speeds, 120 \(\times\) 16/24 = 80. 90 and 75 km come from taking 18 or 15 km/h as the upstream speed.
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