Correct Answer: 4 kmph

Explanation:

Let the distance in one direction = k kms

Speed in still water = 4.5 kmph

Speed of river = 1.5

Hence, speed in upstream = 4.5 - 1.5 = 3 kmph

Speed in downstream = 4.5 + 1.5 = 6 kmph

Time taken by Rajesh to row upwards = k/3 hrs

Time taken by Rajesh to row downwards = k/6 hrs

Now, required Average speed = $$\begin{gathered} Total distance Total speed = 2 k k 3 + k 6 \\ = 2 k x 18 6 k + 3 k = 4 kmph \end{gathered}$$ 

 

Therefore, the average speed of the whole journey = 4kmph.

Correct Answer: 1 kmph

Explanation:

Speed of the boat upstream = 36/9 = 4 kmph

Speed of the boat in downstream = 36/6 = 6 kmph

Speed of stream = 6-4/2 = 1 kmph

Correct Answer: 900 kms

Explanation:

Let the distance he covered each way = d kms

According to the question,

d/45 - d/50 = 1

=> d = 450 kms.

 

Hence, the total distance he covered in his way = d + d = 2 d = 2 x 450 = 900 kms.

Correct Answer: 1 : 3

Explanation:

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.

d w + c = 1 2 d w - c

=> 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