Rate Problem:

"In a bike race, Jake and Kevin started at the same time and rode with constant speeds. Jake rode at 30miles per hour, and Kevin rode at 35miles per hour. If Jake crossed the finish line 10 minutes after Kevin, how many miles long was the race."

You can successfully solve the problem if you set Kevin's time as T and Jake's as T+1/6. However, I had a student ask if we could set Jake as T and Kevin as T-1/6. We went through the problem that way, but it does not work. Ideas? Anyone know the math principle behind this?