Answer
(a) $83\ dB$
(b) $31623$
Work Step by Step
(a) Given $D=80\ dB$, we have $80=10log\frac{I}{I_0}\longrightarrow \frac{I}{I_0}=10^8$. When the intensity is $2I$, we have $D=10log\frac{2I}{I_0}=10log(2\cdot10^8)=80+10log2\approx83\ dB$
(b) Assume we need $x$ number of people, we have $125=10log\frac{xI}{I_0}=10log10^8x=80+10logx$, thus $logx=4.5$ and $x=10^{4.5}\approx31623$
