mass/volume = density
mass = (440 mg)*(1g)/(1000mg) = 0.440g
volume = (1000cm)(1000cm)(t)
where t = thickness
density = 2.70 g/cm^3 = (0.440g)/((1000cm)(1000cm)(t))
multiply both sides by 't' and divide both sides by (2.70g/cm^3)
t = (0.440) / ((1000cm)(1000cm)(2.70)) = 1.629x10^-7 cm
t = (1.629 x 10^-7 cm)*(1000000 micrometers)/(1 cm) = 0.1629 micrometers
Answer is t = 0.1629 micrometers