11
Q:

A motorboat can go 10 miles downstream on a river in 20 min. It takes 30 min for this boat to go back at the same 10 miles. Find the speed of the current ?

A) 8 m/h B) 5 m/h
C) 7 m/h D) 6 m/h

Answer:   B) 5 m/h



Explanation:

As the distance travelled is constant, the time taken is inversely proportional to speed.

Let 'u' be the speed of the current and 'v' be the speed of the boat.

Speed of the boat downstream = v + u & upstream = v - u

=> v+u/v-u = 30/20 => v/u = 5 => v = 5u

=> v + u = 6u = 10/20/60 miles/hr

=> 6u = 30 m/h

=> u = 5 m/h

Q:

A man went downstream for 28 km in a motor boat and immediately returned. It took the man twice as long to make the return trip. If the speed of the river flow were twice as high, the trip downstream and back would take 672 minutes. Find the speed of the boat in still water and the speed of the river flow.

A) 12 km/hr, 3 km/hr B) 9 km/hr, 3 km/hr
C) 8 km/hr, 2 km/hr D) 9 km/hr, 6 km/hr
 
Answer & Explanation Answer: B) 9 km/hr, 3 km/hr

Explanation:

Let the speed of the boat = p kmph

Let the speed of the river flow = q kmph

From the given data,

2 x 28p + q = 28p - q

 

=> 56p - 56q -28p - 28q = 0

=> 28p = 84q

=> p = 3q.

Now, given that if

283q + 2q + 283q - 2q = 67260=> 285q + 28q = 67260=> q = 3 kmph=> x  =3q = 9 kmph

 

Hence, the speed of the boat = p kmph = 9 kmph and the speed of the river flow = q kmph = 3 kmph.

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0 158
Q:

Rajesh rows in still water with a speed of 4.5 kmph to go to a certain place and comes back. Find his average speed for the whole journey, if the river is flowing with a speed of 1.5 kmph?

A) 2 kmph B) 4 kmph
C) 6 kmph D) 8 kmph
 
Answer & Explanation Answer: B) 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 =Total distanceTotal speed = 2kk3 + k6 = 2k x 186k + 3k = 4 kmph 

 

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

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0 146
Q:

A boat takes 2 hours to travel from point A to B in still water. To find out its speed upstream, which of the following information is/are required?

A. Distance between point A and B.

B. Time taken to travel downstream from B to A.

C. Speed of the stream of water.

A) Only A and B B) Only B and C
C) All are required D) Any one pair of A and B, B and C or C and A is sufficient
 
Answer & Explanation Answer: D) Any one pair of A and B, B and C or C and A is sufficient

Explanation:

Let distance between A & B = d km
Let speed in still water = x kmph
Let speed of current = y kmph

from the given data,
d/x = 2

From A) we get d
From B) we get d/x+y
From C) we get y

 

So, Any one pair of A and B, B and C or C and A is sufficient to give the answer i.e, the speed of upstream.

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0 257
Q:

A man can row against the current three fourth of a kilometer in 15 min and returns same distance in 10 min, then ratio of his speed to that of current?

A) 5:1 B) 3:1
C) 4:1 D) 2:1
 
Answer & Explanation Answer: A) 5:1

Explanation:

Let the speed of the man in still water = p kmph

Speed of the current = s kmph

Now, according to the questions

(p + s) x 10 = (p - s) x 15

2p + 2s = 3p - 3s

=> p : s = 5 : 1

 

Hence, ratio of his speed to that of current = 5:1.

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2 301
Q:

A motorboat goes 8 km an hour in still water, but takes thrice as much time in going the same distance against the current than going with the current. Then find the speed of the current?

A) 4 kmph B) 6 kmph
C) 3 kmph D) 2 kmph
 
Answer & Explanation Answer: A) 4 kmph

Explanation:

Let the speed of current = 'C' km/hr

 

Given the speed of boat in still water = 6 kmph

 

Let 'd' kms be the distance it covers.

 

According to the given data,

 

Boat takes thrice as much time in going the same distance against the current than going with the current

 

i.e, d8 - C = 3 × d8 + C

 

 24 - 3C = 8 + C 4C = 16 C = 4 kmph

 

Hence, the speed of the current C = 4 kmph.

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4 362
Q:

 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) 1 : 3 B) 3 : 2
C) 2 : 3 D) 3 : 1
 
Answer & Explanation Answer: A) 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.

dw+c = 12dw-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

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4 274
Q:

If sum of upstream and downstream speed of a boat is 82 kmph, and the boat travels 105 km. upstream in 3 hr, Find the time taken by boat to cover 126 km downstream. 

A) 2.8 hrs B) 2.7 hrs
C) 2.6 hrs D) 2.5 hrs
 
Answer & Explanation Answer: B) 2.7 hrs

Explanation:

Let Speed of boat in still water = b

Let Speed of still water = w

Then we know that,

Speed of Upstream = U = boat - water

Speed of Downstream = D = boat + water

Given, U + D = 82

b - w + b + w = 82

2b = 82

=> b = 41 kmph

From the given data,

41 - w = 105/3 = 35

w = 6 kmph

Now,

b + w = 126/t

=> 41 + 6 = 126/t

=> t = 126/47  = 2.68 hrs.

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5 480
Q:

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) 7:4 B) 11:4
C) 4:7 D) 8:3
 
Answer & Explanation Answer: D) 8:3

Explanation:

Let the speed of the boat upstream be p kmph and that of downstream be q kmph

 

Time for upstream = 8 hrs 48 min = 845hrs

 

Time for downstream = 4 hrs

 

Distance in both the cases is same.

 

=> p x  845= q x 4

 

=> 44p/5 = 4q

 

=> q = 11p/5

 

 Now, the required ratio of Speed of boat : Speed of water current

 

q+p2:q-p2

 

=> (11p/5 + p)/2  : (11p/5 - p)/2

 

=> 8 : 3

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