Math
posted by Jus on .
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!

b,
20=10log(I/Io)
I/Io=antilog2=100
so I is 100 times (Io)
c.
120=10log(I/Io)
I/Io=antilog 12=10^12