1/3 part of a certain journey is covered with the speed of 25 km/hr, 1/2 part of the journey is covered with the speed of 45 km/hr and the remaining part covered with the speed of 37.5 km/hr. What is the average speed (in km/hr) for the whole journey?
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d = total distance
t = total time
d/3 at 25
d/4 at 30
5d/12 at 50
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t = (d/3)/25 + (d/4)/30 + (5d/12)/50
150t = 2d + 5d/4 + 5d/4 = 9d/2d
300t = 9d
t = 3d/100
d = 100t/3
d/t = rate = 100/3
= 33 1/3 km/hr
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