Sunday, January 26, 2014

Basic facts and techniques of Boats and Streams of Quantitative Aptitude

Posted by Jasleen Behl
Boats and Streams is a part of the Quantitative aptitude section. This is just a logical extension of motion in a straight line. One or two questions are asked from this chapter in almost every exam. Today I will tell you some important facts and terminologies which will help you to make better understanding about this topic.

Basic Concept

If direction of boat is same as direction of the stream, then it is known as DOWNSTREAM
and if directions are opposite, then it is known as UPSTREAM. Following figure is representing the same:

i.e. if boat is moving with stream then it is known as Downstream and if opposite to stream, then it is Downstream.

Downstream Speed and Upstream Speed

In case of Downstream, as you can see the direction is same, speeds of stream and boat will be added to get Downstream speed.

If  Speed of boat in still water = u km/hr
Speed of stream = v km/hr, then

Downstream Speed = (u+v) km/hr

Similarly, if I talk about upstream speed, as the direction of boat and stream is opposite, speed of both will be subtracted.

i.e. Upstream Speed = (u-v) km/hr

Study the following figure, notice the directions and try to remember this i.e.
   If directions are same then speeds will be added and
   If directions are opposite then speeds will be subtracted

Speeds of Boat and Stream if Downstream and Upstream Speeds are given

Speed of Boat = `1/2` (Downstream Speed + Upstream Speed)

Speed of Stream = `1/2` (Downstream Speed - Upstream Speed)

Problems with Solution 

Example1: Speed of boat in still water is `5` km/hr and speed of stream is `1` km/hr. Find the downstream speed and upstream speed.

Solution: Given that, u = `5` km/hr

v = `1` km/hr

Downstream speed = u+ v km/hr 
⇒ `5`+ `1`= `6` km/hr

Upstream speed = u -v km/hr
⇒ `5`- `1` = `4` km/hr

Example2: A man takes `3` hours to row a boat `15`km downstream of river and `2` hours `30` min to cover a distance of 5 km upstream. Find speed of river or stream.

Solution: We need to find speed of stream from downstream speed and upstream speed. See how I calculate it:

As You know, Speed = Distance/ Time

So, Downstream Speed = `(15)/3` = `5` km/hr
Upstream Speed = `5/2.5` = `2` km/hr

Now, As i have discussed, Speed of stream = `1/2` (Downstream Speed - Upstream Speed)

⇒Speed of stream = `1/2 (5-2)` 
⇒`3/2` = `1.5` km/hr

Example3: A man can row `7`km/hr in still water. If in a river running at `2` km/hr, it takes him 50 minutes to row to his place and back, how far off is the place?
*Important Question*

Solution: Given, u = `7`km/hr
v = `2`km/hr
From u and v , we can calculate Downstream speed and upstream speed.

Downstream Speed = (u + v) = `7+2` = `9` km/hr
Upstream Speed = (u-v) = `7-2` = `5` km/hr 

Now, we need to find DISTANCE and time is given,

Time = Distance / Speed

Let required distance = x km

Time taken in downstream + Time taken in upstream = Total Time
⇒ `(x/9) + (x/5) = (50)/(60)`........................ 50 minutes = `(50)/(60)` hrs

⇒Calculating the above equation: x = `2.68`km

I hope You have better understanding of this topic now. Also check the following topics:


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