Cant solve this mileage question! PLZ HELP

This is a question from the College board's "Official SAT Study Guide"

I can do most mileage/distance/time problems but I'm stuck on this on, can someone please explain it to me.

Esther drove to work in the morning at an average speed of 45 miles per hour. She returned home in the evening along the same route and an average of 30 miles per hour. If Esther spent a total of one hour commuting to and from work, how many miles did Esther drive to work in the morning.

rate,time.distance problem

posted by razewell

1. RXT=D

2. outbound 45xT =D

3. inbound 30x(1-T)=D

4. 45T = 30x(1-T)

5.T=2/5 hr

6. 45x2/5 =18 miles

I hope this helps you to better understand the solution

bj