In this problem we'll consider the neutron decay. Here are some useful constants:

Neutron mass: $939\mathrm{.}56542$ MeV$/$c$2$

Proton mass: $938\mathrm{.}272089$ MeV$/$c$2$

Electron mass: $0\mathrm{.}51099895$ MeV$/$c$2$

There are three types of neutrino $($and therefore three types of antineutrino$).$ Unfortunately we do not know any of the masses, but we do know that the sum of all three must be less than $0\mathrm{.}120$ eV$,$ or more than one million times lighter than the electron. For that reason, we can take them to be exactly massless for almost all purposes.

As a reminder, here's the decay:

n $->$ p $+$ e$-+$ e

What is the total energy on the left side of the equation?