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Q:

Amith can row a boat d km upstream and the same distance downstream in 5 hours 15 minutes. Also, he can row the boat 2d km upstream in 7 hours. How long will it take to row the same distance 2d km downstream for Amith ?

A) 4 hrs 10 min B) 3 hrs 15 min
C) 3 hrs 30 min D) 4 hrs 1 min

Answer:   C) 3 hrs 30 min



Explanation:

Let the speed of boat and stream be x and y kmph respectively.

According to question

d/x+y + d/x-y = 5h 15m or 21/4 hrs ......(i)

and 2d/x-y = 7...... (ii)

From eq. (i) and (ii)

2d/x+y = 7/2

Hence, Amith will take to row 2d km distance downstream in 7/2 hrs

= 3.5 hrs

= 3 hrs 30 min.

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 =  hrs

Time for downstream = 4 hrs

Distance in both the cases is same.

=> p x  = q x 4

=> 44p/5 = 4q

=> q = 11p/5

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

=> (11p/5 + p)/2  : (11p/5 - p)/2

=> 8 : 3

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6 75
Q:

In one hour, a boat goes 12 km/hr along the stream and 6 km/hr against the stream. The speed of the boat in still water (in km/hr) is:

A) 9 B) 8
C) 7 D) 7.5
 
Answer & Explanation Answer: A) 9

Explanation:

Speed in still water = Average of Speed in Upstream and speed in Downstream

= 1/2 (12 + 6) kmph = 9 kmph.

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6 38
Q:

A motorboat 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 motorboat and speed of the water current respectively?

A) 3:7 B) 7:9
C) 5:4 D) 8:3
 
Answer & Explanation Answer: D) 8:3

Explanation:

Let the man's rate upstream be x kmph and that downstream be y kmph.

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

x x 8 4 = (y x 4)
5
44 x =4y
5
 y = 11 x.
5
 Required ratio = y + x : y - x
2 2
   = 16x x 1 : 6x x 1
5 2 5 2
   = 8 : 3
5 5

   = 8 : 3.

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7 33
Q:

A man can row upstream at 16 km/hr and downstream at 24 km/hr. Find the speed of the current.

A) 4 kmph B) 6 kmph
C) 5 kmph D) 3 kmph
 
Answer & Explanation Answer: A) 4 kmph

Explanation:

Speed of the current = 24-16/2

= 8/2

= 4 km/hr.

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6 57
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 & Explanation 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

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