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Tom drove from P to Q at constant speed 30 km/h and then drove from Q to P at constant speed X km/h. If the average speed of the whole journey is 45 km/h, then what is X?

 Aug 2, 2020
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Call QP = d

 

distance/ rate = time

 

d/30  = time one way       d/ x  =  time return

 

total distance = 2d       total distance / total time  = 45

 

2d / (d/30 + d/x)   = 45

2d / (( dx+30d)/30x) = 45

60 dx /(dx+ 30d) = 45

60x / (x+30) = 45

60 x = 45x + 1350

15x = 1350

x = 90 km/hr

 Aug 2, 2020

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