Q:  What is the mass interior to R = 10 kpc?


          R  =  10 kpc    =  3.08 x 10^(20) m

          v  =  230 km/s  =  2.3 x 10^(5) m/s


                                    v^2 * R
           M(interior)  =  -----------------------------
                             6.67 x 10^(-11) N*m^2/kg^2


                        =   2.4 x 10^(41) kg

                        =   1.2 x 10^(11) solar massses