How would you calculate the mass defect of Cobalt-60 given the following information?

Question

The isotope Cobalt-60 has a nuclear mass of 59.933820 u
Mass of Proton: 1.007825 u
Mass of Neutron: 1.008665 u
1 u = 931.5 MeV

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Answer 1 · 2017-08-16T10:26:48

Well, if the #""^60Co# isotope has a mass of #59.933820*u#.....

We take the PARTICLE masses of #27# #"protons"#, and #33# #"neutrons"#. I am not going to bother with electronic mass.

And so......

#27xx1.007825*u+33xx1.008655*u-underbrace(59.933820*u)_"atomic mass of isotope"#

#=0.56307*u# is the mass defect.

Why did I use #"27 protons"# and #"33 neutrons"#?