Solution
= Solution
At fixed expansion and conversion rates, a smaller neutron half-life reduces $\tau_n$ and hence $X_n(t_N)=X_{n,f}e^{-(t_N-t_f)/\tau_n}$. \b[Fewer neutrons survive, so less helium-4 forms.] If the reduced lifetime arises from a stronger weak coupling, its accompanying faster neutron-proton conversion also delays freeze-out and reinforces this decrease.