An ion with mass number $56$ contains $3$ units of positive charge and $30.4\%$ more neutrons than electrons. Assign the symbol to this ion.

Since $3$ positive charges are present in the ion, the number of electrons is less than the number of protons by $3$.

Let $x$ be the number of electrons.

The number of protons is $x+3$.

The number of neutrons is $x+0.304x=1.304x$.

The mass number is $56$.

It is equal to the number of protons and the number of electrons.

$56=x+3+1.304x$

$2.304x=53$

$x=23$

The number of electrons is $23$.

The number of protons is $23+3=26$.

The number of neutrons is $56-26=30$.

The symbol is ${}_{26}^{56}F{e}^{3+}$.