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if A=mass number

[M][/p]=mass of proton

[M][/n]=mass of neutron

M=mass of nucleus

then packing fraction= [Z[M][/p]+(A-Z)[M][/n]-M]/A

why do some text books and websites write it as

packing fraction=(M-A)/A

where M=mass of nucleons

A=mass number

from what i have understood the numerator could not possibly mean mass defect since mass number is not equal to the total mass of nucleons.

where have i not understood well or is the problem with the text books and the websites (quite unlilkely i might say)