(a) $\ _{0}n^{1} \rightarrow \ _{1}H^{1} + \ _{- 1}e^{0} + \overline{v} + Q$
$\mathrm{\Delta}m = m_{n} - m_{\alpha} - m_{e}$
$= \left( 1.6725 \times 10^{- 27} - 1.6725 \times 10^{- 27} - 9 \times 10^{- 31} \right)kg$
$= - 9 \times 10^{- 31}kg$
Energy $= 9 \times 10^{- 31} \times \left( 3 \times 10^{8} \right)^{2}$
$= 0.511MeV$
Which is nearly equal to $0.73MeV$