JEE Advanced 2020 · Paper 2, Physics question 4

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 $\rho_0 = 1.25\ \mathrm{kg\,m^{-3}}$ and $h_0 = 6000$ m, the value of $N$ is ______.