$a)n_{Fe}=\frac{1.1000}{56}\Rightarrow N_{Fe}=n_{Fe}.N_o=\frac{1000.6,023.10^{23}}{56}$$\Rightarrow N_e=26N_{Fe}\Rightarrow m=m_e.N_e=\frac{9,1.10^{-31}.26.1000.6,023.10^{23}}{56} kg$
$\Rightarrow m\approx 25,447.10^{-11} kg$
$b) 1 kg Fe \rightarrow 25,447.10^{-11} kg electron$
$m_{Fe} \rightarrow 1 kg$
$\Rightarrow m_{Fe}=\frac{1}{25,447.10^{-11}}\approx 3,93.10^{9} kg$