Average velocity from multiple displacement/velocities

[BogMonkey]

Heres the problem:
"A car travels at a constant speed of 60kph for 30km, 40kph for another 30km and 50kph for the final 30km. What is the average speed of the car."

I know that in this case I can just add the velocities and divide by 3 since the displacements are equal and get 50kph as the average speed but when I tried to do it the long way I didn't get the same answer. First I calculated the total time elapsed like this d1/v1 + d2/v2 + d3/v3 and got t = 1.85h. Then to get the average speed I divided the total displacement by time (90/1.85) and got 48.65kph.

there's a pattern there I've tried this with various problems and I always a little less than the right answer. Whats going on here?

 

[Doc Al]

I know that in this case I can just add the velocities and divide by 3 since the displacements are equal and get 50kph as the average speed

Although it might seem right, it's not. You really need to find total displacement over time.

Now if the time intervals were equal, then it would work.