How to determine mobility when two mechanisms exist simultaneously Two scattering mechanisms exist in a semiconductor. If only the first
  mechanism is present, the mobility will be $\pu{250 cm2/V s}$. If only the
  second mechanism is present, the mobility will be $\pu{500 cm2/V s}$.
  Determine the mobility when both scattering mechanisms exist at the
  same time.


I'm not sure what the last sentence is trying to tell me. I know mobility is $q\tau_c/m_n$, and I think I need to solve for mean free time here to help, but I'm not sure what I should use for $m_n$ since I don't know what the semiconductor is, and I'm not given a value. 

Although, since I'm given two mobilities to start with, should I solve for $\tau_c$ twice and average those? Really not sure what direction I should be going in (if only my professor answered his emails).
I needed to add the inverses of the given mobilities and take the inverse of that to get my final result. 

$1/\mu = 1/\mu_L + 1/\mu_I$