Let $\mathrm{a}$ be the number of protons in an element. 
The number of neutrons is

$\begin{array}{ccc}\mathrm{a}+0.32\mathrm{a}& =& 1.32\mathrm{a} \end{array}$
As the mass number is $181$. 
It is equal to the sum of the number of protons and the number of neutrons.
$\begin{array}{ccc}181& =& \mathrm{a} + 1.32\mathrm{a}\\ \mathrm{a}& =& 78\end{array}$

Which is equal to atomic number of $\mathrm{Pt}$.