posted by sam .
A girl starts to walk to her school, which is 20 miles away, at 6:50 and at a rate of 3 miles per hour. After a while, her dad picks her up and drives her through the remainder of the way at 30 miles per hour. If they arrived at school at 9:00, how far did she walk?
Let n be the distance she walks, and 20-n be the distance she rides. Then:
She walked 5 miles; and rode for 15