Answer
23 dB
Work Step by Step
One mosquito emits a sound intensity of $I_o=1\times 10^{-12}W/m^2$.
Two hundred mosquitoes would emit a sound intensity 200 times that of one mosquito.
$$\beta=10log\frac{I}{I_o}=10 log\frac{200I_o}{I_o}= 23 dB$$