Mobility due to lattice and impurity scattering -Example

What is the mobility due to lattice and impurity scattering if the mobility due to lattice scattering only is 250 cm²/V-sec and the mobility due to impurity scattering only is 500 cm²/V-sec?
Answer:

Since the mobility due to two scattering mechanisms is obtained from the inverse of the sum of the inverses of the individual mobilities one finds that

1/m_total = 1/m_lattice + 1/m_impurities = 1/250 + 1/500

so that
m_total = 167 cm²/V-sec.