Question

How many neutrons does element X have if its atomic number is 48 and its mass number is 167?

Answer 1

If its atomic number is 48, then it has 48 protons in the nucleus
of each atom.  Any more mass than that is supplied by the neutrons
that are mixed in there with the protons.

If the mass is 167, and 48 of those are protons, then there are

             (167 - 48)  =  119 neutrons

in each nucleus.