Q) A motor boat whose speed is 18 km/h in still water takes 1 hour more to go 24 km upstream than to return downstream to the same spot. Find the speed of the stream.

Ans: It is given that,

Speed of motor boat = 18 km/hr
Distance travelled =24 km
Time taken to travel in upstream is 1 hour more than in downstream

We need to find out: Speed of the stream

Let’s consider Speed of the stream is X km/ hr

Now, we know that during upstream, stream flows opposite to the boat and hence net speed is difference of the two speeds.
Hence, Net speed in upstream SU = 18 – X
The time taken to flow 24 km Upstream TU = ……. (i)

Similarly, during downstream, stream flows along the boat and hence net speed is sum of the two speeds.
Hence, Net speed in downstream SD = 18 + X
The time taken to flow 24 km downstream TD= ……. (ii)

Given that, TU – TD = 1 24 (18 + X) – 24 (18 – X) = (18 – X)(18 + X)
48 X = 324 – X2
X2 + 48 X – 324 = 0
(X + 54)(X – 6) = 0
X = – 54, X = 6
Since Speed’s value can not be negative, it means X -54, and hence X = 6

Therefore speed of the stream is 6 km/h

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