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A boat running upstream takes 8 hours 48 minutes to cover a certain distance, while it takes 4 hours to cover the same distance running downstream. What is the ratio between the speed of the boat and speed of the water current respectively?

A.

2:1

B.

3:2

C.

8:3

D.

None of these

Answer with explanation

Answer: Option CExplanation

Let the speed of the boat be x kmph and speed of the water current be y kmph.

So, boat’s rate in upstream = x – y

And boat’s rate in downstream = x + y

Then, distance covered upstream in 8 hrs 48 min = Distance covered downstream in 4 hrs.

Or, (x – y) * (44/5) = (x + y) * 4

Or, 44x – 44y = 20x + 20y

Or, 24x = 64y

Or, x/y = 64/24 = 8/3

Or, x : y = 8 : 3

Workspace

A man’s speed with the current is 15 km/hr and the speed of the current is 2.5 km/hr. The man’s speed against the current is:

A.

10 km/hr

B.

9 km/hr

C.

8.5 km/hr

D.

12 km/hr

Answer with explanation

Answer: Option AExplanation

Man’s speed with the current = 15 km/hr

=> speed of the man + speed of the current = 15 km/hr

speed of the current is 2.5 km/hr

Hence, speed of the man = 15 – 2.5 = 12.5 km/hr

man’s speed against the current = speed of the man – speed of the current

= 12.5 – 2.5 = 10 km/hr

Workspace

A man rows to a place 48 km distant and come back in 14 hours. He finds that he can row 4 km with the stream in the same time as 3 km against the stream. The rate of the stream is:

A.

2.5 km/hr

B.

2 km/hr

C.

1.5 km/hr

D.

1 km/hr

Answer with explanation

Answer: Option DExplanation

Workspace

A man takes 3 hours 45 minutes to row a boat 15 km downstream of a river and 2 hours 30 minutes to cover a distance of 5 km upstream. Find the speed of the current.

A.

3 km/hr

B.

4 km/hr

C.

1 km/hr

D.

2 km/hr

Answer with explanation

Answer: Option CExplanation

First of all, we know that

speed of current = 1/2(speed downstream – speed upstream)

So we need to calculate speed downstream and speed upstream first.

Speed = Distance / Time

Workspace

A boat travels upstream from B to A and downstream from A to B in 3 hours. If the speed of the boat in still water is 9km/hr and the speed of the current is 3km/hr, the distance A and B is:

A.

18 kmph

B.

10 kmph

C.

14 kmph

D.

12 kmph

Answer with explanation

Answer: Option DExplanation

Speed downstream = (9+3) km/hr = 12km/hr

Speed upstream = (9-3) km/hr = 6 km/hr

Distance AB = x km

=====>>>> x/6 + x/12 = 3

=====>>>> 2x + x = 36

=====>>>> x= 12

Workspace

A motorboat went the river for 14 km and then up the river for 9 km. If took a total of 5 hrs the entire journey. Find the speed of the river flow if the speed of the boat in still water is 5 km/h.

A.

3 km/hr

B.

5 km/hr

C.

2 km/hr

D.

6 km/hr

Answer with explanation

Answer: Option CExplanation

Let the Speed of the stream be x km/hr

Then, Upward Speed = [ 5 – x ] km/hr and

Downward Speed = [ 5 + x ] km/hr

Workspace

In a river flowing at 2 km/hr, a boat travels 32 km upstream and then returns downstream to the starting point. If its speed in still water be 6 km/hr, find the total journey time.

A.

18 hours

B.

15 hours

C.

12 hours

D.

10 hours

Answer with explanation

Answer: Option CExplanation

speed of the boat = 6 km/hr

Speed downstream = (6+2) = 8 km/hr

Speed upstream = (6-2) = 4 km/hr

Distance travelled downstream = Distance travelled upstream = 32 km

Total time taken = Time taken downstream + Time taken upstream

Workspace

A motorboat takes half time to cover a certain distance downstream than upstream. What is the ratio between rate of current and rate of boat in still water?

A.

3 : 1

B.

2 : 3

C.

3 : 2

D.

1 : 3

Answer with explanation

Answer: Option DExplanation

Let the speed of the boat in still water is ‘w’

Speed of the current is ‘c’

Let the distance between two places is ‘d’

According to the question, motorboat takes half time to cover a certain distance downstream than upstream.

=> 2w – 2c = w + c

=> w = 3c

=> c : w = 1 : 3

Hence, the ratio between rate of current(c) and rate of boat in still water(w) = 1 : 3

Workspace

A man takes twice as long to row a distance against the stream as to row the same distance in favor of the stream. The ratio of the speed of the boat in still water and stream is

A.

1:3

B.

2:4

C.

3:1

D.

4:2

Answer with explanation

Answer: Option CExplanation

Let speed downstream = x kmph

Then Speed upstream = 2x kmph

So ratio will be,

(2x+x)/2 : (2x-x)/2

=> 3x/2 : x/2 => 3:1

Workspace

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?

A.

1 hr 30 min

B.

1 hr 15 min

C.

1 hour

D.

40 minutes

Answer with explanation

Answer: Option BExplanation

Workspace

A Ship of Length 300m traveling from point A to B downstream passes a Ghat along the river in 18 sec, while in return it passes the same Ghat in 24 sec. If the rate of current is 9 Km/hr. Then what is the length of the Ghat?

A.

100 m

B.

80 m

C.

60 m

D.

40 m

Answer with explanation

Answer: Option CExplanation

(S+9)*18 = (S-9)*24

S =63

300+x = 72*5/18*18

x =60

Workspace

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A.

21 kmph

B.

23 kmph

C.

22 kmph

D.

25 kmph

Answer with explanation

Answer: Option CExplanation

Workspace

A boat can travel with a speed of 16 km/hr in still water. If the rate of the stream is 5 km/hr, then find the time taken by the boat to cover the distance of 84 km downstream.

A.

8 hours

B.

6 hours

C.

4 hours

D.

2 hours

Answer with explanation

Answer: Option CExplanation

It is very important to check, if the boat speed given is in still water or with water or against water. Because if we neglect it we will not reach on right answer. I just mentioned here because mostly mistakes in this chapter are of this kind only.

Lets see the question now.

Speed downstream = (16 + 5) = 21 kmph

Time = distance/speed = 84/21 = 4 hours

Workspace

Find the rate of the stream, if a boat covers 120 km downstream and 40 km upstream in 4 hours.

A.

10 km/hr.

B.

15 km/hr.

C.

20 km/hr.

D.

25 km/hr.

Answer with explanation

Answer: Option AExplanation

Distance covered in downstream = 120 km

Time taken in downstream = 4 hours.

Rate of downstream = distance / time = a = 120km / 4 hours = 30 km/hr.

Distance covered in upstream = 40 km

Since it takes same time to cover this 40km,

Time taken in upstream = 4 hours.

Rate of upstream = distance / time = b = 40km / 4 hours = 10 km/hr.

Speed of stream = (a – b)/2 = (1/2)(30 – 10) km/hr = 10 km/hr.

Workspace

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