# Homework Help: Displacement and Velocity Problem

1. Sep 11, 2009

### Loppyfoot

1. The problem statement, all variables and given/known data
A car making a 100-km journey travels 40 km/h for the first 50 km. How fast must it go during the second 50 km to average 50 km/h?

2. Relevant equations

3. The attempt at a solution
I calculated the time to travel the first 50km at 40km/h, to take 1.25 hours. So it take 1.25 hours to drive 50km at 40km/h. So I need to find how fast it must go during the second 50km to average 50km/h. I am confused. Is this a two-part problem?

2. Sep 11, 2009

### tiny-tim

Hi Loppyfoot!

Well, it's a two-part solution

first find what the total time must be to average 50 km/h, and then find how fast the last half has to be.

What do you get?

3. Sep 11, 2009

### Loppyfoot

Alright, I think I have seemed to have gotten it; with the little catalyst you provided.
If the first half take 1.25 hours, the last half must take 0.75 hours. I then used d=rt, and plugged in:
50km = r(0.75hours)
r = 66.67 km/hour

Thanks!