  We derive the average mass profile of elliptical galaxies from the ensemble
of 161 strong gravitational lens systems selected from several surveys,
assuming that the mass profile scales with the stellar mass and effective
radius of each lensing galaxy. The total mass profile is well fitted by a
power-law \rho(r) \propto r^\gamma with best-fit slope \gamma = -2.11+/-0.05.
The decomposition of the total mass profile into stellar and dark matter
distributions is difficult due to a fundamental degeneracy between the stellar
initial mass function (IMF) and the dark matter fraction f_DM. We demonstrate
that this IMF-f_DM degeneracy can be broken by adding direct stellar mass
fraction measurements by quasar microlensing observations. Our best-fit model
prefers the Salpeter IMF over the Chabrier IMF, and a smaller central dark
matter fraction than that predicted by adiabatic contraction models.
