How many times more intense is the full 10 person choir than the school alone?

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.