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 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?