Question

The speed of a boat in still water is 20km/hr. It can go 60km
upstream and return downstream in 8 hours. Find the speed of
4
stream



​

Answer 1

Step-by-step explanation:

Let, the speed of the stream be x km/hr

Speed of boat in still water =20 km/hr

∴Speed of boat with downstream 20+x km/hr

∴ Speed of boat with upstream 20−x km/hr

As per given condition

20−x

48

−

20+x

48

=1

⟹48[

20−x

1

−

20+x

1

]=1

⟹[

(20−x)(20+x)

20+x−20+x

]=

48

1

⟹

400−x

2

2x

=

48

1

⟹96x=400−x

2

⟹x

2

+96x−400=0

⟹x

2

+100x−4x−400

⟹x(x+100)−4(x+100)=0

⟹(x−4)(x+100)=0

Either, x=4 or x=−100

∵ Speed cannot be negative ∴x=4 km/hr is considered.

∴ the speed of the stream =4 km/hr

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