Question

The speed of a motor boat itself is 20 km/h and the speed of the current is 4 km/h. The boat went 120 km downstream. What distance will the boat cover in the same time going upstream?

Answer 1

Time of going down stream is given by (120)/(20+4) =5 h

Then distance in upstream is given by 5×(20-4)=80 km

Answer 2

Given :

Speed of motor boat , v = 20 km/h .

Speed of the current , u = 4 km/h .

Distance travelled by boat downstream , d = 120 km .

To Find :

Distance covered by boat in the same time going upstream .

Solution :

Time taken to go 120 km downstream .

$t=\dfrac{D}{(v+u)}\\\\t=\dfrac{120}{24}\ hr\\\\t=5\ hr$

So , distance covered by boat in the same time going upstream is :

$D=(v-u)t\\\\D=(20-4)\times 5\ km\\\\D=80\ km$

Therefore , Distance covered by boat in the same time going upstream is 80 km .

Learn More :

Relative Velocity

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