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 ρ(h)=ρ0e−h/h0 , where ρ0=1.25 kg m−3 and h0=6000 m , the value of N is ____.