Answer
120 dB = 1W/m$^2$
20 dB is lower in intensity than a 120 dB sound by a factor of 10.
Work Step by Step
According to table 12-2, a 120dB sound has an intensity of $1 W/m^2$.
A sound level increase of 10 dB means that the intensity increases by a factor of 10, so a sound level increase from 20 dB to 120 dB means that the intensity rises by a factor of $10^{10}$.
