Stupid Riddle that is driving me mad

1. Oct 24, 2005

Tom McCurdy

You drive a car at a speed of 40 km/hr to a place and then 60 km/hr back... what is the average speed of the car.

I feel really stupid for asking this but its really making me mad..

2. Oct 24, 2005

Tom McCurdy

hahaha as soon as i posted it it was so obvious...

Set distance equal to one

t1=1/40 t2=1/60

$$\frac{\Delta d}{\Delta t}=avg vel$$

$$\frac{2}{\frac{1}{40}+\frac{1}{60}}$$

3. Oct 24, 2005

Tom McCurdy

Which produces 48 km/hr as answer

4. Oct 25, 2005

TD

You are correct. Often people make the obvious mistake to take the arithmetic mean, being (40+60)/2 = 50 but in this case, you need the harmonic mean

5. Oct 28, 2005

ivybond

harmonic mean could be calculated slightly easier:
$$\frac{v_1 v_2}{\frac{v_1 + v_2}{2}}$$

Last edited: Oct 28, 2005