physics
posted by Toks .
Although these quantities vary from one type of cell to another, a cell can be 2.10 ìm in diameter with a cell wall 50.3nm thick.
If the density (mass divided by volume) of the wall material is the same as that of pure water, what is the mass (in mg) of the cell wall, assuming the cell to be spherical and the wall to be a very thin spherical shell?

surface area of sphere = 4 pi r^2
volume of wall = thickness times surface area = 4 pi r^2 t
mass of wall = density * volume
= rho * 4 pi r^2 t