Ответ:
1. Helen has been washing the dishes for fifteen minutes.
Has Helen been washing the dishes for fifteen minutes?
Helen has't been washing the dishes for fifteen minutes.
2. My classmates have been writing a dictation since the beginning of the lesson.
Have my classmates been writing a dictation since the beginning of the lesson?
My classmates haven't been writing a dictation since the beginning of the lesson.
3. Sam has been driving a car for five hours now.
Has Sam been driving a car for five hours now?
Sam hasn't been driving a car for five hours now.
4. I have been looking for information on the subject since last Friday.
Have I been looking for information on the subject since last Friday?
I haven't been looking for information on the subject since last Friday.
5. Molly has been painting this picture for three days now.
Has Molly been painting this picture for three days now?
Molly hasn't been painting this picture for three days now.
6. The tourists have been doing the sightseeing since nine o’clock.
Have the tourists been doing the sightseeing since nine o’clock?
The tourists haven't been doing the sightseeing since nine o’clock.
7. You have been working as a computer programmer for more than fifteen years.
Have you been working as a computer programmer for more than fifteen years?
You haven't been working as a computer programmer for more than fifteen years.