Answer:
In the symbols for nuclei, the superscript (the upper number) gives the ``mass
number'' of the nucleus, i.e. the number
of nucleons (= number of protons + number of neutrons), while the subscript (the
lower number) gives the ``atomic" number of the nucleus, i.e. the number of
protons. Thus,
(carbon-13) has 6 protons and 13 - 6 = 7 neutrons,
(iron-56) has 26 protons and 56 - 26 = 30 neutrons,
(strontium-90) has 38 protons and 90 - 38 = 52 neutrons,
and (hydrogen-3 = tritium) has 1 proton and 3 - 2 = 2 neutrons.
(note that , and
is an ``antineutrino''.)