suesie970 wrote:

I know that average speed = total distance / total time

But can someone explain another formula I've found on this site:

average speed = (2ab)/(a+b)

a and b represent the average speed of each trip

Where does this come from?

Thanks!

Hey,

This formula is applicable when distances are equal.

example;

Mack travels half of his journey by train at the speed f 120 km/h ad rest half by car at 80 km/h. what is the average speed?

We can solve this problem by many ways.

1. Total distance,take LCM of 120 and 80 =240

therefore,time taken by train=120/120= 1 h

time taken by car = 120/80 = 3/2 h

Average speed = total distance/total time taken

=240/1+3/2

= 96 km/h

2.Method ;

Average speed=2xy/x+y ,where x and y are the speeds

Average speed= 2*120*80/120+80

= 96 km/h

I hope it helps you.