Given that the mass of the proton is 1.007825 u, the mass of the neutron is 1.00866 u, and the nucleus $_{19}^{40} \mathrm{K}$ has 19 protons and 21 neutrons.
Mass of 19 protons = 19 x 1.007825 u = 19.148675 u
Mass of 21 neutrons = 21 x 1.00866 u = 21.18186
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