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?