On a 120km track, a train travels the first 30 km at a uniform speed of 30 km/hr. How fast
must the train travel the next 90 km so as to average 60 km/hr for the entire trip?
20. A train travels at 60 km/hr for 0.52 hr; at 30 km/hr for the next 0.24 hr and at 70 km/hr
for the next 0.71 hr.
What is the average speed of the train?
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A train travels 120km totally and at first 30km it travels at a uniform speedof 30km/hr then it has to travel 90km more at totl average of 60km/hr so as 120-30=90and then it has to travel at average of 90km/hr
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average speed = total distance/total time taken
60 km/h = ( 30 + 90)km/ time taken
60 km/h = 120 km/time taken
time taken = 120 km/60km/h = 2 hours
time taken by train in 1st 30 km = distance/speed = 30/30 = 1hours
hence, rest time = total time - time taken by train in 1st 30 km = 2 - 1 = 1 hours
hence,
speed in next 90 km by train = distance/Time = 90 km/1 hour = 90km/h
20. average speed of train = total distance/ total time taken
total distance =( 60 × 0.52 + 30 × 0.24 + 70 × 0.71 ) km = 88.1 km
total time taken = 0.52 + 0.24 + 0.71 = 1.47 hours
average speed = 88.1/1.47 = 59.93 km/h
≈ 60 km/h
60 km/h = ( 30 + 90)km/ time taken
60 km/h = 120 km/time taken
time taken = 120 km/60km/h = 2 hours
time taken by train in 1st 30 km = distance/speed = 30/30 = 1hours
hence, rest time = total time - time taken by train in 1st 30 km = 2 - 1 = 1 hours
hence,
speed in next 90 km by train = distance/Time = 90 km/1 hour = 90km/h
20. average speed of train = total distance/ total time taken
total distance =( 60 × 0.52 + 30 × 0.24 + 70 × 0.71 ) km = 88.1 km
total time taken = 0.52 + 0.24 + 0.71 = 1.47 hours
average speed = 88.1/1.47 = 59.93 km/h
≈ 60 km/h
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