We know that A = p + n ⇒ 81 = p + n Let number of protons = x According to question, No. of neutrons = x + $\frac{x \times 31.7}{100}$ = 1.317 ∴ x + 1.317x = 81 or 2.317x = 81 ∴ x = $\frac{8}{2.317}$ = 34.96 ≈ 35. Thus , symbol = $\,\mathrm{^{81}_{35}Br}$