At a concert, the loudness of sound, L, in decibels, is given by the equation L=10log(I/I_o), where I is the intensity, in watts per square metre, and I_o, is the minimum intensity of sound audible to the average person, or 1.0 x 10^-12 W/m^2.

a) The sound intensity at a concert is measured to be 0.9 W/m^2. How loud is the concert?

My answer (that is correct); 119.54 dB

b) At the concert, the person beside you whispers with a loudness of 20 dB. What is the whisper's intensity?

My attempt: (but it is wrong)

20+119.54=10log(I/10^-12)

Then I solved for I, to get T=89

c) On the way home from the concert, your car stereo produces 120 dB of sound. What is its intensity?

I did not get this answer either, but tried to do it like b...

Help on parts b and c please