Simple average speed (quick glance should do the trick)

1. Oct 5, 2008

mbisCool

1. The problem statement, all variables and given/known data

A car travels up a hill at a constant speed of 30 km/h and returns down the hill at a constant speed of 66 km/h. Calculate the average speed for the round trip.

2. Relevant equations

avaerage speed= 2*speed1*speed2/(speed1+speed2)
or
average speed=total distance/total time

3. The attempt at a solution

The actual distance is arbitrary and is not given so i first used the second equation with several different distances to check work, all gave me the answer 49.81132075 km/h. The website said this answer was wrong so i tried using the method describeb in equation 1. I got the same answer as I did with the total distance/ total time. You can also work backwards to get the distance chosen at the start. Any insight as to what im doing wrong? and why two different methods give the same answer but are both wrong..

2. Oct 5, 2008

Staff: Mentor

These equations look OK.

But this calculation seems off. Recheck your arithmetic.

3. Oct 5, 2008

mbisCool

sigh I had been using 40km/h instead of 30 as the first speed the entire time :( thanks though