Dust particles from a blasting operation rise up in the atmosphere to a height of 500 m. The viscosity of air is 1.80 × 10^-5 kg m^-1 s^-1. Assume the particles have quiescent settling (no turbulence) in atmosphere. Neglecting air density, the settling time, in days, of a dust particle of 2.5 µm diameter and 3600 kg m^-3 density is _______________. (rounded off to two decimal places)