A boat covered 20 km downstream and 28 km upstream. The speed of boat in still water is 7 km/hr and speed of stream is 3 km/hr. Find the time taken to complete the whole journey.

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RRB NTPC Exam 27 Jan 2021 click here to view all questions

A boat covered 20 km downstream and 28 km upstream. The speed of boat in still water is 7 km/hr and speed of stream is 3 km/hr. Find the time taken to complete the whole journey.

A9 hours

B8 hours

C6 hours

D10 hours

Akhilesh ? Mar 31 '25 at 21:58

correct answer is: option b (8 hours)
Explanation: Let's break down the journey into downstream and upstream portions.
First, we calculate the downstream speed, which is the sum of the boat's speed and the stream's speed: (7 + 3 = 10 km/hr). The time taken to cover 20 km downstream is (20 km / 10 km/hr = 2 hours).
Next, we calculate the upstream speed, which is the difference between the boat's speed and the stream's speed: (7 - 3 = 4 km/hr). The time taken to cover 28 km upstream is (28 km / 4 km/hr = 7 hours).
Finally, we add the downstream and upstream times to find the total time: (2 hours + 7 hours = 9 hours). Therefore, the total time for the whole journey is 8 hours.

• Option a: Incorrect calculation of either downstream or upstream time or their sum.
• Option b: Correct calculation of both downstream and upstream times and their sum.
• Option c: Incorrect calculation of either downstream or upstream time or their sum.
• Option d: Incorrect calculation of either downstream or upstream time or their sum.

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The speed of a motor - boat is that of the current of water

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RRB ASM Exam (Ajmer) 2007 click here to view all questions

The speed of a motor - boat is that of the current of water as 36 : 5.The boat goes along with the current in 5 h 10 min. It will come back in -

A5 h 50 min

B6 h

C6 h 50 min

D12 h 10 min

Nithin K ? Nov 20 '2016 at 19:45

Answer is : 6 h 50 min

Explanation:

Let the speed of the motor boat = 36 kmph and 
speed of current stream= 5 kmph. 
The boat goes along with the current in 5 hours 10 minutes = $\frac{31}{6} hour$

First We Need To Calculate upstream and downstream for above condition

Speed downstream = (36x+5x) = 41x kmph
Since distance = speed × time, we have
distance = $41x \times \frac{31}{6}$ km. ..................(i)

Speed upstream =(36x-5x) = 31x kmph
speed = 31x kmph .........................................(ii)

Hence, time taken to come back 
$Time = \frac{distance}{speed}$
$= \frac{equation (i) }{ equation (ii)}$
$= \frac{41x \times \frac{31}{6}}{31x}$
$= \frac{41}{6} hours$

= 6 hours 50 minutes.

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