There are other greenhouse gasses besides carbon dioxide (in particular water vapor) but in order to give a definite answer, let's pretend the atmosphere is completely free of them. In this case, it's easy to calculate the average temperature of the earth by holding the incoming shortwave heat flux at its present value and assuming that it is balanced by outgoing blackbody radiation from the surface.
Similarly, rather than deal with your 20% carbon dioxide atmosphere explicitly, let's just say the atmosphere provides a "perfect" single-layer greenhouse blanket: an isothermal one that allows shortwave radiation through in the same way the existent one does, but absorbs all the outgoing longwave radiation (I don't think anywhere close to 20% concentration of carbon dioxide would be required to approach this limit). This greenhouse blanket reaches a temperature such that it radiates the same amount of heat to space as is coming in from the sun. Since it radiates equally upward and downward (and we assume all of the downward longwave radiation is absorbed), this means the total energy absorbed (and therefore, in equilibrium, radiated) at the surface is doubled relative to what it would be with no greenhouse blanket. Note that a larger warming effect is possible if the “blanket” is allowed to stratify so that the bottom surface is hotter than the top.
So, with no greenhouse gasses in the atmosphere: $$F_{sun}=\sigma T^4,$$ where $F_{sun}$ is the average, over the whole surface of the earth, of the heat flux absorbed from the sun (roughly $240 W/m^2$), $\sigma$ is the Stefan-Boltzmann constant, $5.67 \times 10^{-8}Wm^{-2}K^{-4}$, and T is the average temperature of the earth's surface. This results in $T=255K=-18^{\circ}C$. Pretty chilly.
With a "perfect" greenhouse atmosphere, $$2F_{sun}=\sigma T^4,$$ which results in $T=303K=30^{\circ}C$, about 15 degrees warmer than the present average. You could reach a slightly higher average surface temperature--perhaps a couple of degrees--by assuming that the ice caps melt, lowering the albedo.