Discussion
What is speed of Ram in still water and speed of river, if Ram covers 20 km in 1 hour swimming downstream and covers the same distance in 2 h swimming upstream?
Answer: D
Write \( b \) for the boat's speed in still water and \( r \) for the stream's. Then
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b + r = \frac{20}{1} = 20 \text{ km/h}, \qquad b - r = \frac{20}{2} = 10 \text{ km/h} \]
\[ b = \frac{20 + 10}{2} = 15 \text{ km/h} \]
\[ r = \frac{20 - 10}{2} = 5 \text{ km/h} \]
Every option lists a pair summing to 20, so the discriminator is the difference: only 15 and 5 give an upstream speed of 10 km/h.
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