A hot air balloon is carrying some passengers, and a few sandbags of mass 1   kg each so that its total…

Initially
$mg=F_b$

$480\times g=V\left({\rho }_0{e}^{-\frac{100}{6000}}\right)g$      ....(1)

Finally $(480-m)g=V\left({\rho }_0{e}^{-\frac{150}{6000}}\right)g$     .....(2)

dividing $\frac{480-m}{480}=e^{-\frac{1}{40}+\frac{1}{60}}$

$m=480\left(1-e^{-1/120}\right)$

Hence $e^{-1/120}\approx 1-\frac{1}{120}$

$1-e^{-1/110}=\frac{1}{120}$

$m=4 \mathrm{kg}$

Hence, four sandbags are thrown.