Past exam of the mathematics course of the University of Cambridge / 2010 / iii / Paper 53 / 2 / i / d / Solution

The equilibrium neutron-to-proton ratio is $e^{-\Delta /(k_{B}T)}$. Increasing the neutron-proton mass difference suppresses neutron abundance at the same freeze-out temperature. Therefore less helium-4 forms, with other inputs held fixed. Any associated shortening of neutron lifetime further reduces survival, rather than reversing this direct effect.