Physics problem I don't understand?

Question

assuming that biological substances are 98% water, estimate the mass of a cell with the diameter of 1.0 um. How would I begin to solve this?

Answer 1

find the area of the circle (cell they r normally round)using 1.0 um then multiply it by 98 and the remaining 2% is the mass of the cell without the water

Answer 2

Since a cell is normally a 3D object, the mass will be the volume times the density.  Forget the 98%; the remaining 2% is close enough to the density of water that the formula m = V*ρ will apply for an ESTIMATE.

V = π*D³/6 = π(1E-6)³/6 = 5.236E-19 m³

ρ = 1000 kg/m³

m = V*ρ = (5.236E-19)*1000 = 5.236E-16 kg