Question

a Boat takes 2 hours to go 40 km down the stream and it returns in 4 hours find the speed of the boat in still water and speed of stream

Answer 1

Hey

Here is your answer,

let the speed of boat is x
and the speed of the stream is y

While going downstream, speed is
=40/2 km/hr
= 20 km/hr.
While going upstream, speed is
= 40/4 km/hr
= 10 km/hr.

Relative speed while going upstream is x + y
Relative speed while going downstream is x - y

Therefore,
x + y = 20
x - y = 10
=> 2x = 30
=> x = 15 km/hr. (answer)
:. y = 5 km/hr. (answer)

Hope it helps!!

Answer 2

Let the speed of boat is x

and the speed of the stream is y

While going downstream, speed speed of the boat is

=>2(x+y) =40

=>x+y= 40/20

=> x+y = 20. ......(i)

While going upstream, speed of the stream is

=> 4(x-y) = 40

=>x-y = 40/10

=> x-y= 10. ......(ii)

adding the equation (i) and (ii),

x + y = 20

x - y = 10

=> 2x = 30

=> x = 15 km/hr.

Now,

x+y = 20

=> 15+y = 20

=> y = 20-15

=> y = 5 km/hr.

Therefore the speed of the boat is 15 km/h and the speed of the stream is 5 km/h.

Hopes it helps you.