JEE Advanced 2020 Physics question

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 . The balloon is floating at an equilibrium height of 100 m. When number of sandbags are thrown out, the balloon rises to a new equilibrium height close to 150 m with its volume remaining unchanged. If the variation of the density of air with height from the ground is , where and m, the value of is .