A hot air balloon is carrying some passengers, and a few sandbags of mass $1 \mathrm{kg}$ each so that its total mass is $480 \mathrm{kg}$. Its effective volume giving the balloon its buoyancy is $V$. The balloon is floating at an equilibrium height of $100 \mathrm{m}$. When $N$ number of sandbags are thrown out, the balloon rises to a new equilibrium height close to $150 \mathrm{m}$ with its volume $V$ remain unchanged. If the variation of the density of air with height $h$ from the ground is $\rho \left(h\right)={\rho }_0{e}^{-\frac{h}{h_0}},$ where ${\mathrm{\rho }}_0=1.25 \mathrm{kg} {\mathrm{m}}^{-3}$ and $h_0=6000 \mathrm{m},$ the value of $N$ is _________.

Hence, the correct answer is 4.