Lisa gets picked up from work by her husband at exactly 4 pm every day. One day Lisa finished work half an hour earlier and started walking to meet up with her husband. She met her husband on the way, and they drove home together. When they arrived home Lisa noticed that they got back 10 minutes earlier than usual.
For how long did Lisa walk?
A few notes:
- Her husband drove from home
- The route is symmetric with regards to speed/distance
- The car can take an instantaneous U-turn
- Lisa didn't tell her husband that she left early
- It's neither more nor less traffic than usual.