Question : Nikunj goes 4 kilometres against the current of a stream in 1 hour, and 2 kilometres along the current in 15 minutes. How long will it take him to go 7 kilometres in stationary water?
Option 1: 1 hour 15 minutes
Option 2: 1 hour 20 minutes
Option 3: 1 hour 10 minutes
Option 4: 1 hour 5 minutes
Correct Answer: 1 hour 10 minutes
Solution :
Let the speed of Nikunj in still water as V (in km/h), and the speed of the stream as S (in km/h).
When Nikunj goes against the current, his effective speed
⇒ V − S = $\frac{4}{1}$ = 4 km/h
When he goes along the current, his effective speed is
⇒ V + S = $\frac{{2}}{\frac{1}4}$ = 8 km/h
Now,
⇒ V − S + V + S = 4 + 8
⇒ 2V = 12
⇒ V = $\frac{12}{2}$ = 6 km/h
Also
Time taken for a distance of 7 km,
⇒ Time = $\frac{\text {Distance}}{\text{Speed}}$ = $\frac{7}{6}$ hours = 1 hour 10 min
Hence, the correct answer is 1 hour 10 min.
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