4120Ca nucleus is formed removing a neutron from 4220Ca.
The mass of 4120Ca plus the mass of a free neutron equals 40.962278u
Mass of neutron =40.962278uu+1.008665u=41.970943u
Difference between 4220Ca plus the mass of a free neutron and the mass of 4220Ca is 0.012321 u.
So, the binding energy of the missing neutron is
(0.012321u)(931.49MeV/u)=11.5MeV