\(K.E\). of the electron = \(mc^2\)-\(m_0c^2\)

But \(m = \frac{{{m_0}}}{{\sqrt {1 - \frac{{{v^2}}}{{{c^2}}}} }} = \frac{{{m_0}}}{{\sqrt {1 - {{\left( {\frac{{0.8\,c}}{c}} \right)}^2}} }} = \frac{{{m_0}}}{{\sqrt {0.36} }} = \frac{{{m_0}}}{{0.6}}\)

Now, \(m{c^2} = \frac{{0.51}}{{0.6}}\) \(MeV = 0.85\  MeV \)

\(\therefore K.E. = (0.85 -0.51)\ MeV = 0.34\ MeV.\)