A hot air balloon is carrying some passengers, and a few sandbags of mass 1 kg each so that its total mass is 480 kg . Its effective volume giving the balloon its buoyancy is $V$. The balloon is floating at an equilibrium height of 100 m . When $N$ number of sandbags are thrown out, the balloon rises to a new equilibrium height close to 150 m with its volume $V$ remaining unchanged. If the variation of the density of air with height $h$ from the ground is $\rho(h)=\rho_0 e^{\frac{h}{h_0}}$, where $\rho_0=1.25 \mathrm{~kg} \mathrm{~m}^{-3}$ and $\mathrm{h}_0=6000 \mathrm{~m}$, the value of $N$ is $\_\_\_\_$ .