jmc

We can start by drawing a diagram of the routes that each of them took: The two routes form a 9-12-15 right triangle. Kelly drove for $9+12=21$ miles at a rate of 42 miles per hour, so she drove for $21/42=.5$ hours which equals 30 minutes. Brenda drove for 15 miles at an average rate of 45 miles per hour, so she drove for $15/45\cdot 60=20$ minutes. Thus, Brenda arrived $30-20=\boxed{10}$ minutes earlier than Kelly.