Imagine that at the time of recombination, the baryonic portion of the universe consisted entirely of 4He (that is, helium with two protons and two neutrons in its nucleus). The ionization energy of helium (that is, the energy required to convert neutral He to He+) is QHe=24.6 eV. At what temperature would the fractional ionization of the helium be X=21? Assume that η=6×10−10 and that the number density of He++ is negligibly small. [The relevant statistical weight factor for the ionization of helium is gHe/(gegHe+)=1/4.]