In cryogenic surgery, a small spherical probe is bought into contact with the diseased tissue which is frozen and thereby destroyed. One such probe can be modeled as a 3-mm diameter sphere whose surface is maintained at 240 K when the surrounding tissue is at 314 K. During the surgical procedure, a thin layer of tissue freezes around the probe at a temperature of 273 K. Assuming that the thermal conductivity of frozen tissue is 1.5 W/m • K and the heat transfer mechanism at the surface is described by an effective convective coefficient of 50 W/m² • K, estimate the thickness of the frozen tissue formed during a 30- minute long operation.