Let $D=140$, just as the problem says. Then solve for $I$ in terms of $I_0$.
Having trouble figuring this out, any help? The decibel level of a sound is given by the equation where I is the sound's intensity and I0 corresponds to the intensity of the weakest sound that can be detected by the human ear. A sound of 140 decibels produces pain in the average human ear. Approximately how many times greater than I0 must the intensity I of a sound be to reach this decibel level?