A boatman goes 80 km upstream and downstream in 6 hours. Time taken to go 5 km downstream is equal to time taken to go 4 km upstream. Find the speed of boat in still water (in km/hr).
Answer & explanation
Correct answer: option 4
Let the speed of boat = x km/hr
the speed of current = y km/hr
Speed of boat in downstream = (x + y) km/hr
Speed of boat in upstream = (x - y) km/hr
Now, ATQ
\(\frac{5}{x + y}\) = \(\frac{4}{x - y}\)
5x - 5y = 4x + 4y
x = 9y
x : y = 9 : 1
Hence,
\(\frac{80}{x + y}\) + \(\frac{80}{x - y}\) = 6
\(\frac{80}{10R}\) + \(\frac{80}{8R}\) = 6
1R = 3
Speed of boat = 9 x 3 = 27 km/hr