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A car completes a trip with an average speed of 50 mph. If the car covered 2/5 of the distance at 60 mph, what was the average speed for the remaining 3/5 of the distance?

 

Please help. 

 May 5, 2022
 #1
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Call the total distance =  D

 

The time for  the  first part of  the  trip  =  (2/5)D / 60   =   D / 150

 

The time  for the second part of the trip   =   (3/5)D / R        where R is the rate we are looking  for

 

And  total  distance   /   total time =   avg rate

 

So

 

D /  [  D/150  + (3/5)D / R ]  =    50

 

1 /  [  1/150  + 3/ (5R) ] =  50

 

1 / [  R/ (150R) + 90 / (150R) ]    = 50

 

150R /  [  90 +R ]   = 50

 

3R  / [ 90 + R ]=   1

 

3R =    90 + R

 

2R =  90

 

R =  45 mph

 

cool cool cool

 May 5, 2022

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