# Finding an Average Speed

zachcumer
Hello!

Here is the question: A reconnaissance plane flies 520 km away from its base at 200 km/h, then flies back to its base at 400 km/h. What is its average speed?

Okay so, 1040 is the total distance. I have to divide that by time...2hrs...I know the answer is roughly 267 km/h. But I cannot remember how to find this!

Thanks for all the help!

Homework Helper
Hi zachcumer,

Hello!

Here is the question: A reconnaissance plane flies 520 km away from its base at 200 km/h, then flies back to its base at 400 km/h. What is its average speed?

Okay so, 1040 is the total distance. I have to divide that by time...2hrs...I know the answer is roughly 267 km/h. But I cannot remember how to find this!

Thanks for all the help!

You are right that you want the total distance divided by the total time to find the average speed. But how did you get that the total time is 2 hours?

(After two hours at the first speed of 200 km per hour, the plane has only traveled 400 km; so it's not even done with the first part of its trip after two hours.)

What do you get for the total time?

zachcumer
Well,
roughly a little more than 2.5??right?

Tedjn
Are you sure that is total time? How are you calculating it?

zachcumer
A reconnaissance plane flies 520 km away from its base at 200 km/h, then flies back to its base at 400 km/h. What is its average speed?

total time: ??

Tedjn
That's what you need to find. Given distance and speed, how would you go about calculating the time?

zachcumer
I know the total distance: 1040. The formula I need to use is D/T...right? I don't know T. I thought it was 2 hrs..?

Tedjn
You need total distance and total time to find the average speed for the entire trip. Conversely, if you knew the average speed and the total distance, you could find the total time. You need to find total time from the more detailed information they give you about the trip. In particular, the trip is broken down into two parts with constant speed in each part. The total time would be the sum of the time it takes to travel each of those parts.

zachcumer
ok so..1040 is the total distance.

200 km/h away from base.

400 km/h coming to base.

200 km/h equals about 5.2 hrs..and so on..

am I on the right track?

Homework Helper
It's on the right track but you have some numbers wrong.

On the flight away from the base: the distance is 520 km, and the speed is 200 km/h. How much time does it take on the flight away?

Coming back to base: the distance is 520 km, and the speed is 400 km/h. How long does it take to come back?

The total time is the sum of these individual times.