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/