Sharmila takes the office bus home every day in the evening. She takes the five o' clock bus and gets off exactly at six o' clock at the bus stop nearest to her house. Her husband driving from home picks her up at the bus stop, and they drive back home. One day Sharmila decided to take the four o' clock bus and reached her bus stop at five o' clock. Thinking that her husband won't be able to pick her up earlier than usual, she decided to walk home. At some point during her walk, her husband picked her up and they reached home ten minutes earlier than they usually reach home. Assuming that Sharmila's husband always drives at a constant speed, and that on this occasion he left home as usual, how long (in minutes) Sharmila walked before she was picked up?