If a person goes 20 km downstream in 5 hours and returns against the stream in 15 hours, then the speed of the stream in km/hr is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(\displaystyle\frac{4}{3}\)
Let u be the speed of the boat in still water and v the speed of the stream (km/hr). Then downstream speed = u + v and upstream speed = u − v.
\( \displaystyle u+v = \frac{20}{5} = 4 \)
\( \displaystyle u-v = \frac{20}{15} = \frac{4}{3} \)
Subtracting the second from the first:
\( \displaystyle 2v = 4-\frac{4}{3} = \frac{8}{3} \)
\( \displaystyle v = \frac{4}{3} \text{ km/hr} \)
Shortcut: Speed of the stream = ½(downstream speed − upstream speed) = ½(4 − 4/3) = 4/3.
CBSE Applied Mathematics (Class XII):
Boats and Streams
