Sun's End-of-Life Density: Comparing to Earth's Water & Atm.

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Near the end of its life, the sun's radius will extend nearly to the distance of the Earth's orbit. I don't understand how to estimate the volume of the sun at that time using the formula for the volume of a sphere (4 pie r^3/3). Using that result, estimate the average matter density of the sun at that time. How does that density compare with the density of water (1g/cm^3)? How does it compare with the density of Earth's atmosphere at sea level (about 10^-3g/cm^3)?
 
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