1. you meet Paul as he is leaving the swimming pool.
-> Have you been swimming?
2. you have just arrived to meet a friend who is waiting for you.
-> Have you been waiting long?
3. you meet a friend in the street. His face and hands are dirty.
-> What have you been doing?
4. a friend of yours is now working in a shop .you want to know how long.
-> How long have you been working there?
5. a friends tells you about his job - he sells computers, you want to know how long.
-> How long have you been selling computers?