12 stamps increased by 125%
\(100\%+125\%=225\%\\\\\\\begin{array}{ccc}12&-&100\%\\\\x&-&225\%\end{array}\ \ \ \ |cross\ multiply\\\\\\100x=12\cdot225\\100x=2700\ \ \ \ |divide\ both\ sides\ by\ 100\\\\\boxed{x=27}\leftarrow answer\)