Answer to Problem II

The sum of the masses of the electron and the proton is less than the mass of the neutron. That is, the total mass of the end products is less than the mass of the original particle. This difference in mass corresponds to the energy released in the decay of the neutron:

Note: The mass that disappears in the decay of the neutron is converted to kinetic energy. Some of this K.E. goes into the resulting motion of the proton and electron: the rest is carried off by a massless particle called the neutrino.