Question

A boy can swim in still water at 4.5 km/h, but takes twice as long to swim upstream than downstream. The speed of the stream is ?
A) 1.8 kmph
B) 2 kmph
C) 2.2 kmph
D) 1.5 kmph

Answer 1

13

Q:

A boy can swim in still water at 4.5 km/h, but takes twice as long to swim upstream than downstream. The speed of the stream is ?

A) 1.8 kmphB) 2 kmphC) 2.2 kmphD) 1.5 kmph

Answer:   D) 1.5 kmph 

Explanation:

Speed of Boy is B = 4.5 kmph

Let the speed of the stream is S = x kmph

Then speed in Down Stream = 4.5 + x

speed in Up Stream = 4.5 - x

As the distance is same,

=> 4.5 + x = (4.5 - x)2

=> 4.5 + x = 9 -2x

3x = 4.5

x = 1.5 kmph

Answer 2

The speed of the stream is 1.5 km/hr

Explanation:

Given:

1. A boy can swim in still water at 4.5 km/h

2. He takes twice as long to swim upstream than downstream

To find:

The speed of the stream

Formula:

Speed downstream = (u+v) km/hr

Speed upstream = (u-v) Km/hr

DOWNSTREAM:

If a person/boat or ship moves along the direction of the stream, it is called downstream.

UPSTREAM:

If a person/boat or ship moves in the opposite direction of the stream, it is called upstream.

Solution:

==> Speed of the boy = 4.5 Km/hr

==> Speed of the stream = x km/hr

==> According to the condition given,

==> downstream = 2(upstream)

==> u = 4.5

==> v = x

==> downstream = (u+v)km/hr

==> downstream = (4.5+x)km/hr

==> upstream = (u-v)km/hr

==> upstream = (4.5-x)km/hr

==> He takes twice as long to swim upstream than downstream

==> 4.5+x = 2(4.5-x)

==> 4.5 + x = 9-2x

==> Separating x and constant

==> 2x + x = 9 - 4.5

==> 3x = 4.5

==> x = 1.5 km/hr

==> The stream speed is 1.5 km/hr