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?

Guest Apr 16, 2022

#1**+1 **

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

BuilderBoi Apr 16, 2022

#2**+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

ElectricPavlov Apr 16, 2022

#4**+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

CPhill Apr 17, 2022