Q. 404.3( 16 Votes )

A train travels the first 15km at a uniform speed of 30 km/h; the next 75 km at a uniform speed of 50 km/h; and the last 10 km at a uniform speed of 20 km/h. Calculate the average speed for the entire train journey.

Answer :

(a) For the first part
In the first part, the train travels 15 km at a speed of 30 km h-1
 The speed of the train is given by
We know the speed of train =30 km/h
              The distance travelled by tain= 15 km 

Time taken by train in the first part, T1=15/30=0.5 h

(b) For the second part
The distance, D= 75 km
The speed, S=50 km/h
Putting the values in the formula to calculate time taken
Time taken, T2=75/50=1.5 h

(c) for the third part
The distance, D=10 km
The speed, S=20 km/h
Putting the values in the formula to calculate time taken
Time taken by train, T3=10/20=0.5 h.

Total distance travelled by train= 15 +75+10 =100 km
Total time taken by train=T1+T2+T3 =0.5 +1.5+0.5=2.5 h

Putting the values in the above formula, we get
Average speed, S=100/2.5=40 km h-1 

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