Verified answer

1. How long have you been making a cake?  I have been making a cake for two hours already.

2. How long has yourfriend been drinking coffee? - Your friend has been drinking coffee for 20 minutes .

3. Have students been reading this text for thirty minutes? - Students have been reading this text for thirty minutes.

4. What has she been singing  the whole evening? -  She has been singing songs the whole evening.

5. Has John been preparing dinner since 3 o'clock? - John has been preparing dinner since 3 o'clock.