Q:  What is the value of mu for neutral hydrogen?


              1    since the mass of one hydrogen atom = 1 hydrogen atom


  Q:  What is the value of mu for neutral helium?


              4    since the mass of one helium atom = 4 hydrogen atoms
  

  Q:  What is the value of mu for a 90:10 mixture of
               hydrogen to helium?


 
                   mass  =  (90 * 1)  +  (10 * 4)
         mu  =    --------------------------------
                   number  =   90     +    10


                    90 + 40
             =    -------------  =  1.3
                    90 + 10