Q:  What is the recombination time?
         Express your answer in seconds and in years.


                               1
        t_r  =  ---------------------------------------------------
                (5.9 x 10^(-25) m^2) (6.7 x 10^5 m/s) (10^7 m^(-3))


             =    2.5 x 10^(11)  seconds   

             =    approx 8000    years