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A car travels the 120 miles from a to b  at 60 miles per hour, and then returns to a on the same road. If the average rate of the round trip is 45 miles per hour, what is the rate, in miles per hour, of the car traveling back from b   to a ?

 Apr 8, 2019
 #1
avatar+18754 
+2

The car travels a total of  120 + 120 = 240 miles

 

rate x time = distance

time = distance/rate

time = 240mi / 45 mi/hr = 240/45 = 5.3333333333333333 hr

 

120 mi / 60 mi/hr = 2 hours for first leg of the trip

Return trip takes   5.33333 - 2 hours = 3.33333 hrs

 

rate times time = distance

rate = distance/time    

rate = 120 / 3.3333 = 36  mi /hr                

 Apr 8, 2019
 #2
avatar+102417 
+1

Here's a fast way to solve this type of problem    :

 

2[ Rate a * Rate b ]

________________   =  Avg rate

  Rate a + Rate b

 

2[ 60* b ]

_________  =   45

    60 + b 

 

120b = 45 [ 60 + b]

 

120b = 2700 + 45b

 

75b = 2700

 

b = 36

 

 

cool cool cool

 Apr 8, 2019

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