if we are talking about sound decibels, then 10 dB equates to 10 times the power. I guess that means 10 times the intensity.
In question A, I guess they are trying to make the point that 10 singers have a sound 10 times as intense as 1 singer, that is, intensities simply add. I don't think this is true in real life, as people can't sing exactly in phase for all notes, and sound waves cancel to some extent. So I don't know how to answer question B, except maybe to assume that intensities simply add.
For question C, that's a simple matter of noticing that 120 dB - 100 dB = 20 dB. But look at how decibels work - the answer is not 20 times. Hint: 30 dB would be 1000 times.