The resistivity of an impurity-free semiconductor at room temperature is p = 50.cm. It becomes equal to

The resistivity of an impurity-free semiconductor at room temperature is p = 50Ω.cm. It becomes equal to Pl = 40Ω ∙ cm when the semiconductor is illuminated with light, and t = 8 ms after switching off the light source the resistivity becomes equal to P2 = 45Ω ∙ cm. Find the mean lifetime of conduction electrons and holes.