a car cover in hilly area a distance of 72 km at a speed of 15m/see and in a plain area distance of 180 km at a speed of 72 km/hr. find the average speed of the whole journey
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1 m/s = 18/5 km/h
15 m/s = 18/5×15 = 54km/h
Time taken in hilly area=distance/speed
= 72km/54km/h
= 4/3 hours
Time taken in plain area=distance/speed
= 180km/72km/h
= 5/2 hours
Average speed=total distance/total time
=72+180km/4/3+5/2 h
=252km/23/6 hours
=252×6/23 = 1512/23
= 68.3 km/h
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