A small isotropic sound source produces an intensity level of 80 dB at a distance of 10 m in free space. Identical, incoherent sources of this kind…

For an isotropic source, $I \propto 1/r^2$. Going from $10\ \mathrm{m}$ to $40\ \mathrm{m}$ reduces the intensity of one source by a factor of $16$, i.e. its level falls by $10\log_{10}16 = 12\ \mathrm{dB}$ (to about $68\ \mathrm{dB}$). Intensities of incoherent sources add, so $N$ sources give $N$ times the intensity, raising the level by $10\log_{10}N$. To restore $80\ \mathrm{dB}$ we need $N = 16$. The value $4$ results from taking $I\propto 1/r$, and $8$ from using the $12\ \mathrm{dB}$ loss as $10\log N$ with $\log 2$ rounding.