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For travel from Washington DC to Baltimore by car, a constant speed of 60 mph was maintained. For the return trip, there was a constant speed \(80\) mph. What was the average speed over the whole two-way trip?

 Apr 16, 2022
 #1
avatar+2455 
+1

Because it's the same distance, it is \((60 + 80) \div 2 = \color{brown}\boxed{70} \) mph. 

 Apr 16, 2022
edited by BuilderBoi  Apr 16, 2022
 #2
avatar+36435 
+4

The time at 60 mph    d/60

The time at 80 mph    d/80

 

Time for the whole trip     2d / a          where a = average

 

d/60 + d/80 = 2d/a

 

a = 2d / ( d/60 + d/80) = 2 / ( 1/60 + 1/80) = 2 / ( 7/240) = 68.57 mph average

 Apr 16, 2022
 #3
avatar+2455 
0

Gahh... Speed is tricky

BuilderBoi  Apr 16, 2022
 #4
avatar+124598 
+1

At first blush, we might assume that the avg speed  =  70  mph

 

However, the avg speed will actually be slower

 

Here's an easy formula  to  figure average speed when the distances are the same  going  and returning

 

Avg speed =    2 (product of the  rates)  /(sum of the rates)   =

 

2 (60 *80)  /( 60 + 80)  =

 

9600  / 140    ≈  68.57 mph

 

 

 

cool cool cool

 Apr 17, 2022

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