To get from her home to a friend's house, Georgia must go past a school, then a post office: Signs tell Georgia that the distance she walks from home to the school is 7/8 mile, and the distance from the school to the post office is 5/6 mile.

The whole trip from home to her friend's house takes Georgia 42 minutes. If Georgia can walk a mile in 18 minutes, then how many miles is the distance from the post office to her friend's house?