What is the speed of a boat going downstream if its speed in still water is 6 km per hour and upstream speed is 5 km per hour?

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Discussion :: Boats and Streams - General Questions (Q.No.11)

11.

A boatman goes 2 km against the current of the stream in 1 hour and goes 1 km along the current in 10 minutes. How long will it take to go 5 km in stationary water?

[A].	40 minutes
[B].	1 hour
[C].	1 hr 15 min
[D].	1 hr 30 min

Answer: Option C

Explanation:

Rate downstream =		1	x 60	km/hr = 6 km/hr.
10

Rate upstream = 2 km/hr.

Speed in still water =	1	(6 + 2) km/hr = 4 km/hr.
2

Required time =		5	hrs = 1	1	hrs = 1 hr 15 min.
4	4

Kas said: (Sep 13, 2011)
How to calculate the required time ?

Something said: (Sep 22, 2011)
Please tell me how to calculate ?

Vikas said: (Oct 6, 2011)
In this, why we calculate average?

Sravankumar said: (Nov 16, 2011)
We caluculate required time by using this formule t=l/s. Here t=5km/4hrs leads to (5/4)km per hour

Narendar said: (May 11, 2012)
We can do this question by another way. Let us suppose speed od boat is we and speed of stream is v1. When boat against the current then speed =v-v1. And when boat along the current then speed =v+v1. v-v1=2/1;. v+v1=1/ (10/60). Calculate the value of v. I got value of v 4km/h. T=5/4. T=1 hr 15 min.

Arun.D said: (Jun 3, 2013)
Upstream=2km (1 hr). Downstream =1km (10 min). For 10 min=1 km. For 1 hr = 6 km. Speed in still water = 1 /2(a + b) km/hr. Speed of still water = 1/2(2+6). = 1/2(8). = 4km/hr. Time = distance/time. Time taken to go 5 km in stationary water = (5/4). = 1hr 25min.

Abhisek said: (Nov 15, 2013)
How is downstream 6km/hr?

Anjali said: (Jan 22, 2014)
See here. Since downstream = 1km (10mins). So for 10 mins = 1km. So from this when we convert to hrs we get. 10/60 =1 km. And that 60 goes to rhs it will be multiplied with kms. i.e., 10=1*60. So 1 hr = 6 kms.

MohitRaj said: (Jul 16, 2015)
We know that, Upstream speed = u-v; Downstream speed=u+v;. According to formula, s=d/t. u-v = 2/1, and u+v = 60/10. On solving these equations, we get. u=4 and v=2. Therefore, speed of boatman = 4 km/hr. Hence, t=d/s. t=5/4. We get 1hr 15min is answer.

Suchita said: (Aug 9, 2015)
I am still confuse for conversion of time. Please somebody can help me to understand all types of conversion of time. Here they have divided in some problem they have multiplied it. Here, 3/1060 and in one example 1812/60, why like this?

Lohith said: (Aug 30, 2015)
s = d/t; u-v = 2/1; u-v = 2. u+v = 2/10/60. u+v = 12. u = 7. v = 5. Final value = 5/7 which is less than a hour.

Sowmya said: (Nov 2, 2015)
I can't understand, please help me.

Anu said: (Jan 11, 2016)
How can rate upstream be 2 km/hr?

Darth Vadar said: (Feb 20, 2016)
Divide 5 by 4 we get 0.25. So why the answer is 1 hr 15 min instead of 1 hr 25 min.

Rohini said: (Jul 17, 2016)
The question says a boatman goes 2 km against the current of the stream in 1 hour which means upstream so, it is 2km/hr.

Rajesh Rapelly said: (Jan 2, 2017)

Let Speed of the boat is 'u' and Speed of the Stream is 'v'. Then u+v=6km/hr ///Speed of the boat in downstream. Then u-v=2km/hr ////Speed of the boat in upstream. Here we need the speed of the boat in Still Water which is 'u', because the speed of stream is zero in still water i.e;v=0. By solving u+v =6 & u-v=2 ,we get u=4km/hr.

Ao, Speed of the boat in Stillwater is 4km/hr, then to travel 5km it take 1hr 15mins.

Kiran said: (Jul 2, 2017)
The ans must be 1hr 25mins. 5/4=1.25. How it can be 1hr 15mins?

Sonia said: (Dec 16, 2017)
1hr = 60mins. So, 1/4 of 1 hr = 1/4*60= 15min.

Manideep said: (Jan 18, 2019)
Thank you @Sonia.

Mayur said: (Jan 19, 2019)

Speed = Distance /Time. Upstream Speed = 2/1 = 2 km/hr. Downstream Speed = 1KM/10Min = 1/(10/60)hr = 1/(1/6) Km/Hr = 6 Km/Hr. Now, Speed in Still Water = 1/2 (a+b) a = Upstream speed means the speed of the boat against the current of the stream. b. = Downstream speed means boat speed along the stream = 1/2(2+6) after simplifying and is 4 Km/hr. Total time required to travel distance of 5km for a boat in stationary water will be? Time = Distance / Speed ----> Formula. = 5/4 -> 4 km/hr is the speed of the boat in stationary water & 5 Km be the distance. = 1.25 hr => 1 hr 15 min.

25/100 *60 = 15 for a minute to hour conversation.

Thomas said: (Sep 6, 2019)
I also agree, The answer should be 1h and 25 mins.

Ibrana said: (Sep 25, 2019)
Please discuss the proper way to find required time.

Varshith said: (Apr 9, 2020)
Thanks @Mayur.

Kessy said: (May 26, 2020)
Anyone, please explain this step 1/2 times (6 + 2).

ABI said: (Jun 4, 2020)
@Kessy Simply need to find the speed of the boat. You have downstream and upstream speed and, Downstream speed=Speed of boat +Speed of stream. 6=x+y---------> 1 Upstream speed=Speed of boat-Speed of stream 4=x-y------------> 2 Solve both 1&2 equations and get x that is the speed of the boat.

Anandh said: (Sep 28, 2022)

@All. It's very simple. Speed of the downstream S = D/T. D= 1km, T = 10mint, mint convert to hour. 10/60 hour then, S = 1km / (10/60)hr, = 60/10 km/hr. Downstream speed s = 6 km/hr then, Upstream speed S = D/T. = 2km / 1hr = 2km / hr. Speed in still water = 1/2 ( Down S + Up S ) 1/2 ( 6+2 ) = 4km / hr. So we need T? For stationary water, D is given 5km ... Then Speed in still water = 4km /hr. T = D / S = 5/ 4 = 1 1/4 hours.

= 1hr 15 mint.