During his morning commute to work in rush hour traffic, Justin's average speed was 30 mi/h. During his afternoon commute back home along the same route, his average speed was 60 mi/h. What was his average speed for the entire round trip?

ATTEMPT:

Okay, so I figured that you would have the same distance right?

So all you need to do is take an average of 30, and 60???

Which is 45???

Could somebody check my work?

Thank you!

\(tommarvoloriddle\)

tommarvoloriddle Nov 9, 2019

#1**+1 **

Not quite, Tom......

The average rate = 2 * product of the rates 2 * 30 * 60 3600

____________________ = ___________ = _____ = 40 mph

sum of the rates 30 + 60 90

P.S.....I can prove this if you want.....

CPhill Nov 9, 2019

#3**0 **

Can you prove it?

I got a teensy bit confused...

IRRELEVANT SORRY I GET DISTRACTED!

tommarvoloriddle
Nov 9, 2019

#4**+1 **

OK ....call the distance one way, D

So....the round trip = 2D

The time one way is D /30

The time back is D /60

So.....the average rate = Total Distance 2D

____________ = ____________

Total Time D/30 + D/60

If you simplify this.....you should get 40 mph

CPhill Nov 9, 2019

#6**+2 **

Actually, I believe it should be **40mph**. If we let the distance be 60 miles, than it takes Justin 2 hrs to work, and 1 hr back home. That sums up to a total of 3 hrs for a distance of 2*60 = 120 miles. Dividing 120 by 3 gives us an answer of **40mph**.

ThatOnePerson Nov 10, 2019