A tank has two rooms separated by a membrane. Room A has $1 \mathrm{~kg}$ of air and a volume of $0.5 \mathrm{~m}^{3}$; room $\mathrm{B}$ has $0.75 \mathrm{~m}^{3}$ of air with density $0.8 \mathrm{~kg} / \mathrm{m}^{3}$. The membrane is broken, and the air comes to a uniform state. Find the final density of the air.