a train moves wiith speed of 30 km/h for 12 min. and next8 min. with speed of 45km/h.the average speed of train is?
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30 km/hr=30*5/18=25/3 m/sec.
45 km/hr=45*5/18=25/2 m/sec.
Total distance = time*speed=(25/3)*12*60 + (25/2)*8*60=6000+6000 m=12 km.
Average speed = Total Distance/Total Time= 12/ (12/60)+ (8/60) = 12/(20/60)
= 12/(1/3) = 36 km/hr.
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