if the number of neutrons are changed in an atom, it makes an isotope of that atom. In an atom number of protons is equal to number of electrons.
we can find the number of neutrons by subtracting atomic number from the mass of an atom.
here atomic mass of iron isotope is 55 and we know from periodic table that atomic number of iron is 26 so 55 -26 = 29
there are 29 neutrons.