Solution
= Solution
Once deuterium survives photodissociation, nuclear reactions rapidly assemble nearly every available neutron into tightly bound helium-4. Protons are more abundant, so neutrons are the limiting constituent. If $r=n_n/n_p=1/7$, then
$$
n_{\rm He}\simeq\frac{n_n}{2}.
$$
Neglecting the neutron--proton mass difference,
$$
Y_p=\frac{\rho_{\rm He}}{\rho_B}
\simeq\frac{4m_p(n_n/2)}{m_p(n_n+n_p)}
=\frac{2r}{1+r}.
$$
Thus
$$
\boxed{Y_p\simeq\frac14.}
$$