John and Priya went for a small picnic. After having their lunch Priya insisted to travel in a motor boat. The speed of the motor boat was 20 km/hr. Priya being a Mathematics student wanted to know the speed of the current. So she noted the time for upstream and downstream. She found that for covering the distance of 15 km the boat took 1 hour more for upstream than downstream.
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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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