Aptitude - Boats and Streams - Discussion

Each of the questions given below consists of a question followed by three statements. You have to study the question and the statements and decide which of the statement(s) is/are necessary to answer the question.


What is the speed of the boat in still water?


The speed downstream is 12 kmph.


The speed upstream is 4 kmph.


In a to and fro journey between two points, the average speed of the boat was 6 kmph.

[A]. I and II only
[B]. All I, II and III
[C]. III, and either I or II
[D]. Any two of the three
[E]. None of these

Answer: Option D


From I and II, speed of boat in still water = 1 (12 + 4) km/hr = 8 km/hr.

From II and III, we get:

Using average speed = 2xy , we get: 2 x 4 x y = 6
x + y 4 + y

8y = 24 + 6y

y = 12.

Required speed = 1 (12 + 4) km/hr = 8 km/hr.

Similarly, I and III also give the answer.

Correct answer is (D).

Tarandeep said: (Nov 26, 2012)  
How this formula for Avg Speed is calculated?

Amit said: (Nov 9, 2013)  
Avg. speed = Total distance / Total time = 2L / t1 + t2.

Time = distance / speed so, t1 = L / x and t2 = L / y.

t1 + t2 = L(x+y) / xy.
Total time = L(x+y) / xy.
Total distance = 2L.

Avg. speed = 2xy/x+y.

Aman said: (Dec 3, 2013)  
Can anybody tell me how we get the value 4 in that average speed formula you have putted.

Aashit said: (Jun 25, 2014)  
Why has 4 been substituted for 'x' when 4=x+y?

Rahul said: (Feb 15, 2015)  
x is the speed of the boat upstream, and y is the speed of the boat downstream.

Swarnashree said: (Aug 28, 2015)  
Why x is taken as speed of the boat upstream?

Shashi said: (Oct 22, 2016)  
How is statement 3 related to question?

There is no need of 3rd statement or no relation b/w 3rd to another statement.

Harshavardan said: (Jan 10, 2017)  
My Analysis.

Upstream is 4kmps and downstream is 12kmps.
Boat speed still in water =(down stream+upstream)/2 = (1/2) * (12+4) = 8kmps.
Boat speed against the water =(down stream-upstream)/2 = (1/2) * (12-4) = 4kmps.
So, Statments 1&2 are efficient to get this answer.
Answer is Option "A".

Priya said: (May 6, 2018)  
Therefore the answer is 0.
But unfortunately, the answer is not 0.
Because any answer cannot be 0.

So, we increment the answer by 1 to avoid any problems.
Therefore the answer is 1.

Abhilasha said: (Jul 17, 2018)  
3rd statement itself is enough as [(u+v)+(u-v)]/2=6.

Which gives u = 6; where u is the speed of the boat in still water and v is the speed of the stream.

Kapadiya Sagar said: (Jun 26, 2020)  
Options 1 and 3 become same.

a+b = 6kmph.

So with one equation two unknown is not solvable.

So, the Correct option is [A].

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