Ava’s father, George, picks her up at the train station when she comes home from school, then George drives Ava home. They always return home at 5:00 pm. One day Ava left school early and got to the train station an hour early. She then started walking home. George left home at the usual time to pick Ava up, and they met along the route between the train station and their house. George picked Ava up and then drove home, arriving at 4:48 pm. For how many minutes had Ava been walking before George picked her up?