An element with mass number $81$ contains $31.7\%$ more neutrons as compared to protons. Assign the atomic symbol.

Let $x$ be the number of protons.

The number of neutrons is $x+0.317x=1.317x$.

The mass number is $81$.

It is equal to the sum of the number of protons and the number of neutrons.

$81=x+1.317x=2.317x$

$x=35$

Hence, the symbol is ${}_{35}^{81}Br$.