Atom consists of electrons, protons and neutrons. If the mass attributed to neutron is halved

and that attributed to the electrons is doubled, the atomic mass of  ${ }_6{C}^{12}$ would be approximately 

No change by doubling mass of electron, however by reducing mass of neutron to half, total atomic mass becomes 6 +3 instead of 6 + 6. Now atomic mass = $\frac{9}{12}\times 100=75\%$

$\therefore$Reduction in atomic mass = 25 %