$(15$ pts $.)$ A spherical balloon with a radius of $0\mathrm{.}5$ m was released at sea level at $25C$ and $1$ atm $.$ What will be the radius of the balioon when it has risen to its maximum altitude where the temperature and the pressure inside the balloon are $-30C$ and $6\mathrm{.}5{10}^{-3}$atm, respectively.?

$1$atm$=101325Pa,$ Volume of a sphere $=\frac{4}{3}{r}^3$