How do I find the average speed for this problem?

[ixerr]

Homework Statement

I don't know how to start.. someone please give me a step-by-step explanation.
Julie drives 100 mi to Grandmother's house. On the way there she drives half of the distance at 42 mi/hr and half the distance at 68 mi/hr. What is Julie's average speed (in mi/hr) on the way to Grandmother's house?

Homework Equations

The Attempt at a Solution

 

[NascentOxygen]

You know the total distance, so you need to work out the total time taken. To do this, work out the travel time on each different section, and then add them up.

If you do it this way, you'll avoid falling into the trap !