An element with  mass number  81 contains $31.7\mathrm{\%}$ more neutrons as compared  to protons. Find the symbol  of the atom

Mass number of the element = 81
$\text{ i.e., }\mathrm{p}+\mathrm{n}=81$
Let the number of protons be x.
Number of neutrons $=\mathrm{x}+\frac{31.7}{100}\times \mathrm{x}=1.317\mathrm{x}$
$\therefore \mathrm{x}+1.317\mathrm{x}=81$ or $2.317\mathrm{x}=81$
or $\mathrm{x}=\frac{81}{2.317}=34.958\approx 35$
Symbol of the element ${=}_{35}^{81}\mathrm{Br}$