Let the number of protons(p) = x ∴ Number of electrons (e) = x – 3 (because the ion carries 3 units of positive charge, it will have 3 electrons less than the number of protons.) Number of neutrons(n) = (x - 3) + $\frac{(x-3)30.4}{100}$ = $\frac{100(x-3)+(x-3)30.4}{100}$ n = $\frac{100x-300+30.4x-91.2}{100}$ = $\frac{130.4x-391.2}{100}$ We know that A = p + n 56 = x + $\frac{130.4x-391.2}{100}$ or 56 = $\frac{100x+130.4x-391.2}{100}$ or 5600 = 230.4x – 391.2 or 5600 + 391.2 = 230.4x or 5991.2 = 230.4x ∴ x = $\frac{5991.2}{230.4}$ = 26 Thus, number of protons (p) = 26, number of electrons (e) = 26 – 3 = 23 therefore, symbol of the ion = $\,\mathrm{^{56}_{26}Fe^{3+}}$