The density of air in a child’s balloon 20 cm in diameter is roughly the same as the den...

The density of air in a child’s balloon 20 cm in diameter is roughly the same as the density of air at sea level, 1019 particles/cm³. To how large a diameter would you have to expand the balloon to make the gas inside the same density as the interstellar medium, about 1 particle/cm3? (Hint:The volume of a sphere is