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A boatman goes 2 km against the current of the stream in 1 hour and goes 1 km along the current in 10 minutes. How long will it take to go 5 km in stationary water?

Solution

Correct Option: 3

Against current (upstream):

22 km in 11 hour = 22 km in 6060 minutes

Rate upstream = 22 km/hr

Along current (downstream):

11 km in 1010 minutes = 66 km/hr

Rate downstream = 66 km/hr

If upstream speed = UU = speed in still water - speed of current = xyx - y

And downstream speed = DD = speed in still water + speed of current = x+yx + y

Then:

xy=2x - y = 2 ... (1)

x+y=6x + y = 6 ... (2)

Adding equations:

2x=82x = 8

x=4x = 4

Therefore, speed in still water = 44 km/hr

For 55 km in still water:

Time = 54\frac{5}{4} hours = 1.251.25 hours = 11 hour 1515 minutes

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