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Averaging Speeds of Individual Laps

```Date: 12/12/2003 at 03:00:45
From: Jaggu
Subject: Average Speed

A driver drives four laps around a track at 10, 20, 30, and 60 km per
hour.  What is his average speed?
```

```
Date: 12/12/2003 at 08:09:00
From: Doctor Luis
Subject: Re: Average Speed

Hi Jaggu,

The average speed is defined as

average speed = total distance / total time

Suppose the length of the track is L km. Then the total distance is

total distance = 4*L km

But how do we find the total time?  If he drives L km at a speed of 10
km per hour, then the trip takes

time = distance / speed

= (L km) / (10 km/hr)

= (L/10) hours

So the total time is

total time = ((L/10) + (L/20) + (L/30) + (L/60)) hours

When you divide those two quantities, the unknown track length L
should cancel out, leaving you with the average speed.

I hope this helped!  Let us know if you have any more questions.

- Doctor Luis, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
Middle School Equations
Middle School Word Problems

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