Discussion
Ratio of upstream speed to that of downstream speed is 3 : 5. If speed of boat in still water is 24 km/hr then in how much time boat will cover 36 km is upstream and 60 km is downstream together ?
Answer: A
The time taken is 4 hours.
The speed in still water is the average of the upstream and downstream speeds. With the ratio 3 : 5, write them as 3k and 5k:
\[ \begin{aligned} \frac{3k + 5k}{2} &= 24,\ k = 6 \\ \text{up} &= 18,\ \text{down} = 30\ \text{km/h} \\ t &= \frac{36}{18} + \frac{60}{30} = 2 + 2 = 4 \end{aligned} \]
5 hours and 3 hours do not follow from these speeds, and since 4 hours is offered, 'None of these' is wrong.
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