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Question

On a 100 km track ,a train travels the first 30km at a uniform speed of 30km per hour. How fast must the train travel the next 70km so as to average 40km per hour for the entire trip

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Solution

Given:

Distance traveled in first part d1 = 30 km
Distance traveled in second part = d2 = 70 km

We know,

Time=DistanceSpeed

So for the first part Time(t1)=3030=1hour

Now,

We know,

Averagespeed=TotaldistanceTotaltime=d1+d2t1+t2

Putting all the values

40=1001+t2

Or, t2=1.5h

Now.

Speed for the second part = Speed=DistanceTime=701.5

After solving

Speed for the second part = 46.67 km/h



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