111. Solving a chemistry problem

In whole cow´s milk, the average volume diameter of fat droplets ranges from 2.9 to 4.4 microns, depending on individual conditions. Other characteristics are more constant: at 20 °C fat density is 0.920 g/cm3, dispersion medium density (skimmed milk) is 1.034 g/cm3, viscosity is 1.60 mPa*s.
Let us assume that whole milk with a fat droplet diameter of 4 µm is placed at 20°C in a rectangular cardboard "package" 15 cm high. At the initial moment of time, the droplets are uniformly distributed in height. How long will it take for 50% of the fat to float to the surface of the milk if all the droplets have the same diameter? (Note that under these conditions, the ascent time for 50% fat is the same as the time for one drop to rise from half the height of the bag)