Question

a boat goes 15 km upstream in 1hr 20 mins speed of the stream is 4 km/hr find the speed of the boat in still water

Answer 1

let speed of boat be x and sream speed : 4km/hr
total speed in upstream = x-4
total distance =15
time =distance /speed
4/3 =15/x-4
=15.25 km/hr

Answer 2

The speed of the boat in still water is 15.25 km/hr.

Given: A boat goes 15 km upstream in 1 hr 20 mins and the speed of the stream is 4 km/hr.

To Find: The speed of the boat in still water.

Solution:

• If the velocity of an object in still water and the velocity of the object swimming upstream are given then, the velocity of the stream can be found by the relation,

                 Vmr = Vm + Vr                                                   ...(1)

Where Vmr = Velocity of the object in still water, Vm = Velocity of an object swimming upstream, and Vr = Velocity of the stream.

Coming to the numerical, we are given;

The velocity of the stream is (Vr) = 4 km/hr

The distance traveled upstream by boat = 15 km

The time is taken by him = 1 hr 20 min = 4/3 hrs

The upstream velocity ( Vm ) of the swimmer = 15 / ( 4 / 3 ) km/hr

                                                                = 45/4 km/hr

So, putting respective values in (1), we get;

                   Vmr = Vm + Vr          

             ⇒   Vmr  =  45/4 + 4

             ⇒   Vmr =  ( 45 + 16 ) / 4

                        = 61/4 km/hr

                        = 15.25 km/hr

Hence, the speed of the boat in still water is 15.25 km/hr.

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