Simple average speed (quick glance should do the trick)

[mbisCool]

Homework Statement

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.

Homework Equations

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

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 I am doing wrong? and why two different methods give the same answer but are both wrong..

 

[Doc Al]

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

These equations look OK.

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.

But this calculation seems off. Recheck your arithmetic.

 

[mbisCool]

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