thermal conductivity

A 3/16 in. thick acrylic testing box with dimensions of 10.0 in.
x 10.0 in. is covered with an unknown 0.50 in. insulation material.
Determine the thermal conductivity for the insulating material if a
25.0W bulb is used to heat the box. The bulb maintains the inside
temperature at 10.0ºC higher than the outside temperature.