**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 S_{U} = 18 – X

The time taken to flow 24 km Upstream T_{U} = ……. (i)

Similarly, during downstream, stream flows along the boat and hence net speed is sum of the two speeds.

Hence, Net speed in downstream S_{D} = 18 + X

The time taken to flow 24 km downstream T_{D}= ……. (ii)

Given that, T_{U }– T_{D} = 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**