It is known that atoms contain protons, neutrons and electrons. If the mass of neutron is assumed to be half of its original value whereas that of electron is assumed to be twice of this original value.

The atomic mass of ${ }_6{\mathrm{C}}^{12}$ will be

A = P + n                        A = Z + n

A = 12                             Z = 6

n = 12 - 6                        n = 6

If  the mass of neutrons becomes half then no. of neutrons = 3

And atomic mass = 6+3 =9

$\begin{array}{l}=\frac{9}{12}\times 100=75\mathrm{\%}=100-75\\ =25\mathrm{\%}\mathrm{less}\mathrm{mass}\end{array}$