Formula to Calculate Baryon/Photon Ratio

Does anyone know how to derive the following formula?

Nb/Ny=(Omega-b*Ey)/(Omega-y*Eb)

Whe
Omega-b is Omega Baryon
Ey is the energy of a CMB photon = 2.7kT, where T=2.725
Omega-y is the ratio of CMB radiation density to the critical density
Eb=Mb*c^2, where Mb is the mass of a baryon


[[Mod. note -- This at least appears like it might be a university
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