A hot air balloon is carrying some passengers, and a few sandbags...

Initially
$mg = {F}_{b}$

$480 \times g = V\left({ρ}_{0}{e}^{- \dfrac{100}{6000}}\right)g$ ....(1)

Finally $( 480 - m ) g = V\left({ρ}_{0}{e}^{- \dfrac{150}{6000}}\right)g$ .....(2)

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

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

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

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

$m = 4kg$

Hence, four sandbags are thrown.