How do scientists determine the number of neutrons in an isotope of an atom?
a.
they find the number of protons.
b.
they divide the atomic mass by two.
c.
they add the number of electrons and protons.
d.
they subtract the atomic number from the atomic mass.

D.  The atomic mass in amu is basically the number or nuclei since the mass of the electrons is negligible.  For a given atom (element) the number of protons is fixed.  Say the element has 10 protons.  If the atomic weight is 14 atomic mass units (amu), you know that there are 4 neutrons, since both neutrons and protons are 1 amu each and there are 10 protons